It works

Content.Client/Input/ContentContexts.cs

@@ -69,6 +69,7 @@ public static void SetupContexts(IInputContextContainer contexts)
             human.AddFunction(ContentKeyFunctions.OpenBackpack);
             human.AddFunction(ContentKeyFunctions.OpenBelt);
             human.AddFunction(ContentKeyFunctions.OfferItem);
+            human.AddFunction(ContentKeyFunctions.ToggleStanding);
             human.AddFunction(ContentKeyFunctions.MouseMiddle);
             human.AddFunction(ContentKeyFunctions.ArcadeUp);
             human.AddFunction(ContentKeyFunctions.ArcadeDown);

Content.Client/Options/UI/Tabs/KeyRebindTab.xaml.cs

@@ -184,6 +184,7 @@ void AddCheckBox(string checkBoxName, bool currentState, Action<BaseButton.Butto
             AddButton(ContentKeyFunctions.MoveStoredItem);
             AddButton(ContentKeyFunctions.RotateStoredItem);
             AddButton(ContentKeyFunctions.OfferItem);
+            AddButton(ContentKeyFunctions.ToggleStanding);
 
             AddHeader("ui-options-header-interaction-adv");
             AddButton(ContentKeyFunctions.SmartEquipBackpack);

Content.Server/Standing/LayingDownComponent.cs

@@ -0,0 +1,17 @@
+namespace Content.Server.Standing;
+
+[RegisterComponent]
+public sealed partial class LayingDownComponent : Component
+{
+    /// <summary>
+    ///     Movement speed multiplier when not standing.
+    /// </summary>
+    [DataField]
+    public float DownedSpeedMultiplier = 0.15f;
+
+    [DataField]
+    public TimeSpan Cooldown = TimeSpan.FromSeconds(2.5f);
+
+    [DataField]
+    public TimeSpan CooldownUntil = TimeSpan.Zero;
+}

Content.Server/Standing/LayingDownSystem.cs

@@ -0,0 +1,94 @@
+using Content.Shared.ActionBlocker;
+using Content.Shared.Input;
+using Content.Shared.Movement.Systems;
+using Content.Shared.Popups;
+using Content.Shared.Standing;
+using Robust.Shared.Input.Binding;
+using Robust.Shared.Player;
+using Robust.Shared.Timing;
+
+namespace Content.Server.Standing;
+
+// Unfortunately cannot be shared because some standing conditions are server-side only
+public sealed class LayingDownSystem : EntitySystem
+{
+    [Dependency] private readonly ActionBlockerSystem _actionBlocker = default!;
+    [Dependency] private readonly MovementSpeedModifierSystem _movement = default!;r
+    [Dependency] private readonly SharedPopupSystem _popups = default!;
+    [Dependency] private readonly Shared.Standing.StandingStateSystem _standing = default!; // WHY IS THERE TWO DIFFERENT STANDING SYSTEMS?!
+    [Dependency] private readonly IGameTiming _timing = default!;
+
+    public override void Initialize()
+    {
+        CommandBinds.Builder
+            .Bind(ContentKeyFunctions.ToggleStanding, InputCmdHandler.FromDelegate(ToggleStanding, handle: false, outsidePrediction: false))
+            .Register<LayingDownSystem>();
+
+        SubscribeLocalEvent<LayingDownComponent, StoodEvent>(DoRefreshMovementSpeed);
+        SubscribeLocalEvent<LayingDownComponent, DownedEvent>(DoRefreshMovementSpeed);
+        SubscribeLocalEvent<LayingDownComponent, RefreshMovementSpeedModifiersEvent>(OnRefreshMovementSpeed);
+    }
+
+    public override void Shutdown()
+    {
+        base.Shutdown();
+
+        CommandBinds.Unregister<LayingDownSystem>();
+    }
+
+    private void DoRefreshMovementSpeed(EntityUid uid, LayingDownComponent component, object args)
+    {
+        _movement.RefreshMovementSpeedModifiers(uid);
+    }
+
+    private void OnRefreshMovementSpeed(EntityUid uid, LayingDownComponent component, RefreshMovementSpeedModifiersEvent args)
+    {
+        if (TryComp<StandingStateComponent>(uid, out var standingState) && standingState.Standing)
+            return;
+
+        args.ModifySpeed(component.DownedSpeedMultiplier, component.DownedSpeedMultiplier);
+    }
+
+    private void ToggleStanding(ICommonSession? session)
+    {
+        if (session is not { } playerSession)
+            return;
+
+        if ((playerSession.AttachedEntity is not { Valid: true } uid || !Exists(uid)))
+            return;
+
+        if (!TryComp<StandingStateComponent>(uid, out var standingState) || !TryComp<LayingDownComponent>(uid, out var layingDown))
+            return;
+
+        var success = ToggleStandingImpl(uid, standingState, layingDown, out var popupBranch);
+
+        // If successful, show popup to self and others. Otherwise, only to self.
+        _popups.PopupEntity(Loc.GetString($"laying-comp-{popupBranch}-self", ("entity", uid)), uid, uid);
+
+        if (success)
+        {
+            _popups.PopupEntity(Loc.GetString($"laying-comp-{popupBranch}-other", ("entity", uid)), uid, Filter.PvsExcept(uid), true);
+
+            layingDown.CooldownUntil = _timing.CurTime + layingDown.Cooldown;
+        }
+    }
+
+    private bool ToggleStandingImpl(EntityUid uid, StandingStateComponent standingState, LayingDownComponent layingDown, out string popupBranch)
+    {
+        var success = layingDown.CooldownUntil <= _timing.CurTime;
+
+        // Note: &= leads to the right-hand side being evaluated regardless of what's on left-hand, so we use normal assignment here instead.
+        if (_standing.IsDown(uid, standingState))
+        {
+            success = success && _standing.Stand(uid, standingState, force: false);
+            popupBranch = success ? "stand-success" : "stand-fail";
+        }
+        else
+        {
+            success = success && _standing.Down(uid, standingState: standingState, playSound: true, dropHeldItems: false);
+            popupBranch = success ? "lay-success" : "lay-fail";
+        }
+
+        return success;
+    }
+}

Content.Shared/Input/ContentKeyFunctions.cs

@@ -55,6 +55,7 @@ public static class ContentKeyFunctions
         public static readonly BoundKeyFunction ZoomIn = "ZoomIn";
         public static readonly BoundKeyFunction ResetZoom = "ResetZoom";
         public static readonly BoundKeyFunction OfferItem = "OfferItem";
+        public static readonly BoundKeyFunction ToggleStanding = "ToggleStanding";
 
         public static readonly BoundKeyFunction ArcadeUp = "ArcadeUp";
         public static readonly BoundKeyFunction ArcadeDown = "ArcadeDown";

Resources/Locale/en-US/escape-menu/ui/options-menu.ftl

@@ -137,6 +137,7 @@ ui-options-function-swap-hands = Swap hands
 ui-options-function-move-stored-item = Move stored item
 ui-options-function-rotate-stored-item = Rotate stored item
 ui-options-function-offer-item = Offer something
+ui-options-function-toggle-standing = Toggle standing
 ui-options-static-storage-ui = Lock storage window to hotbar
 
 ui-options-function-smart-equip-backpack = Smart-equip to backpack

Resources/Locale/en-US/movement/laying.ftl

@@ -0,0 +1,7 @@
+laying-comp-lay-success-self = You lay down.
+laying-comp-lay-success-other = {THE($entity)} lays down.
+laying-comp-lay-fail-self = You can't lay down right now.
+
+laying-comp-stand-success-self = You stand up.
+laying-comp-stand-success-other = {THE($entity)} stands up.
+laying-comp-stand-fail-self = You can't stand up right now.

Resources/Prototypes/Entities/Mobs/Species/base.yml

@@ -310,6 +310,7 @@
   - type: FireVisuals
     alternateState: Standing
   - type: OfferItem
+  - type: LayingDown
 
 - type: entity
   save: false

Resources/keybinds.yml

@@ -258,6 +258,9 @@ binds:
 - function: OfferItem
   type: State
   key: F
+- function: ToggleStanding
+  type: State
+  key: R
 - function: ShowDebugConsole
   type: State
   key: Tilde