From the Admin Dashboard, expand Early Years and select Daily Diaries.
Expand the relevant child's diary then the 'What else...' section and click the + next to Sleeping Times.
Enter the time(s) as below
You can add multiple sleep times by clicking the + again and delete an entry by clicking the red x. The diary will autosave every 10 seconds.
What the parent sees
This is how the above section is displayed on the Parent Portal.