Generally depends on 2 things...

1) When the hell I got to sleep the night before

and

2) When the hell do I have to be in work today?

Usually depending on my shift pattern I'm awake pretty early On a 9am shift I like to wake up at around 6 - 6:30am this is because I like the hour and a half to get showered, shaved, ready and to sit down and munch a bowl of cereal and a cup of tea with time to spare to check emails or something silly. If it's an 8am push this back to around 5:30 - 6:00. If I'm working late shifts I generally won't be home til 9:30pm - 10:00 and have to be in at 12:00 next day so I'll end up waking at 8 - 9am. I hate nights like tonight, I finished at 9pm got home about 10:30 due to delays on the tube and have to be up for work and in by 9am. My stupid body refuses to switch off at times like this and so I end up with 2 - 3 hours sleep.