I think both times I've done it it has been in the morning (around 11 oclock). It doesn't really matter what time of day it is, it just matters what time you wake up. There's no point in booking it for 9 am when you only went to bed at 3. Likewise, there'd be no point in booking it for the evening if you woke up like 6 am ish (the reason being that your 'sleepiness', or rather the levels of the chemical that makes you sleepy (can't remember the name atm, we covered it last year) increases from the time you wake up.)