diff --git a/.github/workflows/code_check.yml b/.github/workflows/code_check.yml
index fca6aa4..0658e20 100644
--- a/.github/workflows/code_check.yml
+++ b/.github/workflows/code_check.yml
@@ -8,6 +8,24 @@ on:
     types: [opened, synchronize, reopened]
 
 jobs:
+  phpstan:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: 7.4
+
+      - name: Install dependencies
+        run: composer install --prefer-dist --no-progress --no-suggest
+
+      - name: Run PHPStan
+        run: vendor/bin/phpstan analyse -c phpstan.neon
+
   phpunit:
     runs-on: ubuntu-latest
 
diff --git a/composer.json b/composer.json
index 084241e..802e032 100644
--- a/composer.json
+++ b/composer.json
@@ -10,7 +10,8 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^9.6",
-        "friendsofphp/php-cs-fixer": "^3.58"
+        "friendsofphp/php-cs-fixer": "^3.58",
+        "phpstan/phpstan": "^1.11"
     },
     "autoload": {
         "psr-4": {
diff --git a/composer.lock b/composer.lock
index 5b0945f..7f3f4c5 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "e960c827904bd4589fdbba562be69885",
+    "content-hash": "742b713e90945bc6ddfaa70138c1c1fc",
     "packages": [
         {
             "name": "guzzlehttp/guzzle",
@@ -1399,6 +1399,64 @@
             },
             "time": "2022-02-21T01:04:05+00:00"
         },
+        {
+            "name": "phpstan/phpstan",
+            "version": "1.11.4",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/phpstan/phpstan.git",
+                "reference": "9100a76ce8015b9aa7125b9171ae3a76887b6c82"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/phpstan/phpstan/zipball/9100a76ce8015b9aa7125b9171ae3a76887b6c82",
+                "reference": "9100a76ce8015b9aa7125b9171ae3a76887b6c82",
+                "shasum": ""
+            },
+            "require": {
+                "php": "^7.2|^8.0"
+            },
+            "conflict": {
+                "phpstan/phpstan-shim": "*"
+            },
+            "bin": [
+                "phpstan",
+                "phpstan.phar"
+            ],
+            "type": "library",
+            "autoload": {
+                "files": [
+                    "bootstrap.php"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "description": "PHPStan - PHP Static Analysis Tool",
+            "keywords": [
+                "dev",
+                "static analysis"
+            ],
+            "support": {
+                "docs": "https://phpstan.org/user-guide/getting-started",
+                "forum": "https://github.com/phpstan/phpstan/discussions",
+                "issues": "https://github.com/phpstan/phpstan/issues",
+                "security": "https://github.com/phpstan/phpstan/security/policy",
+                "source": "https://github.com/phpstan/phpstan-src"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/ondrejmirtes",
+                    "type": "github"
+                },
+                {
+                    "url": "https://github.com/phpstan",
+                    "type": "github"
+                }
+            ],
+            "time": "2024-06-06T12:19:22+00:00"
+        },
         {
             "name": "phpunit/php-code-coverage",
             "version": "9.2.31",
diff --git a/phpstan.neon b/phpstan.neon
new file mode 100644
index 0000000..d8a2bce
--- /dev/null
+++ b/phpstan.neon
@@ -0,0 +1,6 @@
+parameters:
+    level: 6
+    paths:
+        - src
+        - tests
+
diff --git a/src/Client.php b/src/Client.php
index 42b044e..9bab78c 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -19,11 +19,6 @@ class Client
      */
     private GuzzleClient $httpClient;
 
-    /**
-     * @var string API access token
-     */
-    private string $accessToken;
-
     /**
      * @var string Base URI for the Oura API
      */
@@ -49,7 +44,6 @@ public function __construct(string $accessToken, string $apiVersion = 'v2')
                 'Accept' => 'application/json',
             ],
         ]);
-        $this->accessToken = $accessToken;
         $this->apiVersion = $apiVersion;
     }
 
@@ -58,7 +52,7 @@ public function __construct(string $accessToken, string $apiVersion = 'v2')
      *
      * @param string $method HTTP method (GET, POST, etc.)
      * @param string $endpoint API endpoint
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      * @throws RuntimeException if the request fails
      */
@@ -80,7 +74,7 @@ private function makeRequest(string $method, string $endpoint, array $params = [
     /**
      * Retrieves heart rate data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getHeartRate(array $params = []): ResponseInterface
@@ -91,7 +85,7 @@ public function getHeartRate(array $params = []): ResponseInterface
     /**
      * Retrieves personal info data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getPersonalInfo(array $params = []): ResponseInterface
@@ -102,7 +96,7 @@ public function getPersonalInfo(array $params = []): ResponseInterface
     /**
      * Retrieves tag data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getTag(array $params = []): ResponseInterface
@@ -113,7 +107,7 @@ public function getTag(array $params = []): ResponseInterface
     /**
      * Retrieves enhanced tag data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getEnhancedTag(array $params = []): ResponseInterface
@@ -124,7 +118,7 @@ public function getEnhancedTag(array $params = []): ResponseInterface
     /**
      * Retrieves workout data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getWorkout(array $params = []): ResponseInterface
@@ -135,7 +129,7 @@ public function getWorkout(array $params = []): ResponseInterface
     /**
      * Retrieves session data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getSession(array $params = []): ResponseInterface
@@ -146,7 +140,7 @@ public function getSession(array $params = []): ResponseInterface
     /**
      * Retrieves daily activity data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getDailyActivity(array $params = []): ResponseInterface
@@ -157,7 +151,7 @@ public function getDailyActivity(array $params = []): ResponseInterface
     /**
      * Retrieves daily sleep data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getDailySleep(array $params = []): ResponseInterface
@@ -168,7 +162,7 @@ public function getDailySleep(array $params = []): ResponseInterface
     /**
      * Retrieves daily SPO2 data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getDailySpo2(array $params = []): ResponseInterface
@@ -179,7 +173,7 @@ public function getDailySpo2(array $params = []): ResponseInterface
     /**
      * Retrieves daily readiness data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getDailyReadiness(array $params = []): ResponseInterface
@@ -190,7 +184,7 @@ public function getDailyReadiness(array $params = []): ResponseInterface
     /**
      * Retrieves sleep data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getSleep(array $params = []): ResponseInterface
@@ -201,7 +195,7 @@ public function getSleep(array $params = []): ResponseInterface
     /**
      * Retrieves sleep time data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getSleepTime(array $params = []): ResponseInterface
@@ -212,7 +206,7 @@ public function getSleepTime(array $params = []): ResponseInterface
     /**
      * Retrieves rest mode period data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getRestModePeriod(array $params = []): ResponseInterface
@@ -223,7 +217,7 @@ public function getRestModePeriod(array $params = []): ResponseInterface
     /**
      * Retrieves ring configuration data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getRingConfiguration(array $params = []): ResponseInterface
@@ -234,7 +228,7 @@ public function getRingConfiguration(array $params = []): ResponseInterface
     /**
      * Retrieves daily stress data
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getDailyStress(array $params = []): ResponseInterface
@@ -246,7 +240,7 @@ public function getDailyStress(array $params = []): ResponseInterface
      * Retrieves tag data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getTagById(string $documentId, array $params = []): ResponseInterface
@@ -258,7 +252,7 @@ public function getTagById(string $documentId, array $params = []): ResponseInte
      * Retrieves enhanced tag data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getEnhancedTagById(string $documentId, array $params = []): ResponseInterface
@@ -270,7 +264,7 @@ public function getEnhancedTagById(string $documentId, array $params = []): Resp
      * Retrieves workout data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getWorkoutById(string $documentId, array $params = []): ResponseInterface
@@ -282,7 +276,7 @@ public function getWorkoutById(string $documentId, array $params = []): Response
      * Retrieves session data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getSessionById(string $documentId, array $params = []): ResponseInterface
@@ -294,7 +288,7 @@ public function getSessionById(string $documentId, array $params = []): Response
      * Retrieves daily activity data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getDailyActivityById(string $documentId, array $params = []): ResponseInterface
@@ -306,7 +300,7 @@ public function getDailyActivityById(string $documentId, array $params = []): Re
      * Retrieves daily sleep data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getDailySleepById(string $documentId, array $params = []): ResponseInterface
@@ -318,7 +312,7 @@ public function getDailySleepById(string $documentId, array $params = []): Respo
      * Retrieves daily SPO2 data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getDailySpo2ById(string $documentId, array $params = []): ResponseInterface
@@ -330,7 +324,7 @@ public function getDailySpo2ById(string $documentId, array $params = []): Respon
      * Retrieves daily readiness data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getDailyReadinessById(string $documentId, array $params = []): ResponseInterface
@@ -342,7 +336,7 @@ public function getDailyReadinessById(string $documentId, array $params = []): R
      * Retrieves sleep data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getSleepById(string $documentId, array $params = []): ResponseInterface
@@ -354,7 +348,7 @@ public function getSleepById(string $documentId, array $params = []): ResponseIn
      * Retrieves sleep time data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getSleepTimeById(string $documentId, array $params = []): ResponseInterface
@@ -366,7 +360,7 @@ public function getSleepTimeById(string $documentId, array $params = []): Respon
      * Retrieves rest mode period data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getRestModePeriodById(string $documentId, array $params = []): ResponseInterface
@@ -378,7 +372,7 @@ public function getRestModePeriodById(string $documentId, array $params = []): R
      * Retrieves ring configuration data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getRingConfigurationById(string $documentId, array $params = []): ResponseInterface
@@ -390,7 +384,7 @@ public function getRingConfigurationById(string $documentId, array $params = [])
      * Retrieves daily stress data by document ID
      *
      * @param string $documentId Document ID
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface PSR-7 Response object
      */
     public function getDailyStressById(string $documentId, array $params = []): ResponseInterface
@@ -402,7 +396,7 @@ public function getDailyStressById(string $documentId, array $params = []): Resp
      * Retrieves all pages of data for a given endpoint
      *
      * @param string $endpoint API endpoint
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllPages(string $endpoint, array $params = []): array
@@ -428,7 +422,7 @@ public function getAllPages(string $endpoint, array $params = []): array
     /**
      * Retrieves all heart rate data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllHeartRate(array $params = []): array
@@ -439,7 +433,7 @@ public function getAllHeartRate(array $params = []): array
     /**
      * Retrieves all workout data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllWorkout(array $params = []): array
@@ -450,7 +444,7 @@ public function getAllWorkout(array $params = []): array
     /**
      * Retrieves all tag data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllTag(array $params = []): array
@@ -461,7 +455,7 @@ public function getAllTag(array $params = []): array
     /**
      * Retrieves all enhanced tag data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllEnhancedTag(array $params = []): array
@@ -472,7 +466,7 @@ public function getAllEnhancedTag(array $params = []): array
     /**
      * Retrieves all session data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllSession(array $params = []): array
@@ -483,7 +477,7 @@ public function getAllSession(array $params = []): array
     /**
      * Retrieves all daily activity data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllDailyActivity(array $params = []): array
@@ -494,7 +488,7 @@ public function getAllDailyActivity(array $params = []): array
     /**
      * Retrieves all daily sleep data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllDailySleep(array $params = []): array
@@ -505,7 +499,7 @@ public function getAllDailySleep(array $params = []): array
     /**
      * Retrieves all daily SPO2 data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllDailySpo2(array $params = []): array
@@ -516,7 +510,7 @@ public function getAllDailySpo2(array $params = []): array
     /**
      * Retrieves all daily readiness data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllDailyReadiness(array $params = []): array
@@ -527,7 +521,7 @@ public function getAllDailyReadiness(array $params = []): array
     /**
      * Retrieves all sleep data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllSleep(array $params = []): array
@@ -538,7 +532,7 @@ public function getAllSleep(array $params = []): array
     /**
      * Retrieves all sleep time data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllSleepTime(array $params = []): array
@@ -549,7 +543,7 @@ public function getAllSleepTime(array $params = []): array
     /**
      * Retrieves all rest mode period data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllRestModePeriod(array $params = []): array
@@ -560,7 +554,7 @@ public function getAllRestModePeriod(array $params = []): array
     /**
      * Retrieves all ring configuration data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllRingConfiguration(array $params = []): array
@@ -571,7 +565,7 @@ public function getAllRingConfiguration(array $params = []): array
     /**
      * Retrieves all daily stress data, handling pagination
      *
-     * @param array $params Query parameters
+     * @param array<string,string|int> $params Query parameters
      * @return ResponseInterface[] Array of PSR-7 Response objects
      */
     public function getAllDailyStress(array $params = []): array