Your Leaf will track your sleep automatically and display it when you sync in the morning. In case your Leaf has gotten either your bedtime or wake up time wrong and you edit the sleep times manually, the app will overwrite any awake periods during the night with light sleep. This is the case because your input supersedes the data that the Leaf has tracked and the app will assume that the Leaf has tracked this inaccurately and input your own data.
But in the future versions of the Bellabeat app you should be able to edit your sleep and keep the awake periods during the night.