One of the first things you should do when you've got smart lights in your house is to set up a "good night" command for your voice assistant. Alexa and Google Home both support the creation of "Routines", which are a set of commands to your smart devices, based on either a phrase, or you can set them to a schedule.
We use a "Good night" routine to turn all the lights off, except for one or two that we keep on the dimmest setting in case any of the kids get up in the night. If you've got other smart devices, such as smart plugs or connected heaters, you can also add them to the routine.
Using Alexa, the way to do this is to tap "More" in the bottom right corner, and select "Routines". Create a new routine by tapping the white cross in the top right corner. Give the routine a name, and define when you want the routine to happen. We use a "Voice" command, which is "Alexa, Good night", but you can be more creative if you want! You can then add actions such as switching off the lights, and anything else you want to include. It's nice to add a little message from Alexa, like "That was a great day!".
Google Home works in a very similar way, although has some fun pre-set routines you can play with. Adding actions works by just typing in what you'd normally say to the assistant, for example "Turn off all the lights". You may need to test out what works first.
Enjoy your evening routines!