Many people tend to forget to change their mode from day to night before they sleep, and unable to track their Sleep Activity. But don't worry, you'll be able to enter your Sleep Activity manually.
Accessing To Log Sleep Menu
Go to the Right Hand side Menu, and tap on Log Sleep.
Once in the Log Sleep Screen, you can Add your Sleep manually.
Just select In Bed At .... and Awake At .... and Save. That's it! You've manually add your sleep. Do remember to change the mode tonight :)