At precisely the time my body says "sleep is possible at this moment"
if earlier - I toss and turn and cannot sleep, often wake up early and then need a bloody nap at midday.
Factors affecting sleep are:
Whether I have worked that day, or not, how much that week and the hours.
How much sleep I got the day (or previous 2 days before)
Beer consumed, if any, on the night (day) in question
Pot consumed, if any, on the night (day) in question (this stuff helps immensely though, especially the sleepy kind...)
Caffeine consumed and how many hours before sleepy time.
How horny I am and how long since that's been taken care of...(or not).
Whether I am angry or not, the degree of that, and how far I've let my imagination go with relation to it.