diff --git a/NightLight.Core.Tests/FakeHome.fs b/NightLight.Core.Tests/FakeHome.fs
index e7c0112..c7bffd9 100644
--- a/NightLight.Core.Tests/FakeHome.fs
+++ b/NightLight.Core.Tests/FakeHome.fs
@@ -44,6 +44,8 @@ type FakeLight(light: Light) =
             color <- newColor
 
 type FakeHome(now: DateTime) =
+    let mutable time = now
+
     let mutable nightLightStateMachine = NightLightStateMachine now
 
     let assertIsOkAndGet result =
@@ -93,6 +95,8 @@ type FakeHome(now: DateTime) =
         commands |> Seq.iter processCommand
         nightLightStateMachine <- newState
 
+    member _.Time = time
+
     member _.LightStates = friendlyNameToFakeLight.Values |> Seq.map _.LightWithState
 
     member _.Interact(interaction: Interaction) =
@@ -109,4 +113,23 @@ type FakeHome(now: DateTime) =
             |> ReceivedZigbeeEvent
             |> sendEvent
         | HumanInteraction(LightTurnedOff light) -> friendlyNameToFakeLight[light.FriendlyName].TurnOff()
-        | TimeChanged time -> time |> Event.TimeChanged |> sendEvent
+        | TimeChanged newTime ->
+            time <- newTime
+            newTime |> Event.TimeChanged |> sendEvent
+
+type FakeHome with
+    member this.Interact(interactions: Interaction seq) = interactions |> Seq.iter this.Interact
+
+    member this.ForAllLightsThatAreOn condition =
+        this.LightStates
+        |> Seq.choose (fun (light, state) ->
+            match state with
+            | On(brightness, color) -> Some(light, brightness, color)
+            | Off -> None)
+        |> Seq.forall condition
+
+    member this.IsDay() =
+        this.Time.TimeOfDay >= TimeSpan.FromHours 5.5
+        && this.Time.TimeOfDay < TimeSpan.FromHours 20.5
+
+    member this.IsNight() = not (this.IsDay())
diff --git a/NightLight.Core.Tests/NightLightTests.fs b/NightLight.Core.Tests/NightLightTests.fs
index b9a361e..e74a2a0 100644
--- a/NightLight.Core.Tests/NightLightTests.fs
+++ b/NightLight.Core.Tests/NightLightTests.fs
@@ -1,16 +1,29 @@
 namespace NightLight.Core.Tests
 
+open System
 open FsCheck.Xunit
+open FsCheck.FSharp
 
 [<Properties(Arbitrary = [| typeof<Arbitraries> |])>]
 type NightLightTests() =
     [<Property>]
-    let ``Brightness should always be under 255`` (fakeHome: FakeHome) (interactions: Interaction list) =
-        interactions |> Seq.iter (fun interaction -> fakeHome.Interact interaction)
+    let ``Brightness should always be under 255`` (now: DateTime) (interactions: Interaction list) =
+        let fakeHome = FakeHome now
+        fakeHome.Interact interactions
+        fakeHome.ForAllLightsThatAreOn(fun (_, brightness, _) -> brightness < 255uy)
 
-        fakeHome.LightStates
-        |> Seq.choose (fun (_, state) ->
-            match state with
-            | On(brightness, _) -> Some brightness
-            | Off -> None)
-        |> Seq.forall (fun brightness -> brightness < 255uy)
+    [<Property>]
+    let ``Lights should be red during the night`` (now: DateTime) (interactions: Interaction list) =
+        let fakeHome = FakeHome now
+        fakeHome.Interact interactions
+
+        fakeHome.IsNight()
+        ==> fakeHome.ForAllLightsThatAreOn(fun (_, _, color) -> color = Red)
+
+    [<Property>]
+    let ``Lights should be white or yellow during the day`` (now: DateTime) (interactions: Interaction list) =
+        let fakeHome = FakeHome now
+        fakeHome.Interact interactions
+
+        fakeHome.IsDay()
+        ==> fakeHome.ForAllLightsThatAreOn(fun (_, _, color) -> color = White || color = Yellow)