Their sleeping hours...

Valt :

9-12 hours! Can sleep without a trouble but has a trouble waking up.

Shu :

8-9 hours! Wakes up early and sleep early too! Perfect human being that unfortunately didn't exist in this damn world (emotional damage- ;-;)

Lui :

4-6 hours! Can stay awake all night doing his work out!

Free :

Sleeps all day and feels sleepy always. (This being is me- <3 )