I used to have -the- worst time when sleeping 10pm to 6am... I just never ever felt rested at all. I was irritable and angry and frumpy. But, a little of that was due to my mega-low metabolism, but a great deal is just my natural sleep cycle.
Now I sleep somewhere around 8am-noon, and 5pm-9pm on workdays. I've never felt better o_o I've been sleeping a leetle bit heavier lacking my meds for the metabolism (thyroid, really), bit I still feel happier, stay awake at work completely and just generally feel more energenic. Night shift is perfect work for my deranged sleep habits.
Humans have a hard time rousing me. Alarms are the middle range. Strange thuds, sharp noises (metal) and any animal noise usually wakes me right up. It's save two mice my cat caught so far, and Sinistar the rat when she escaped as a baby. Nobody can eat the Sin!
But, on a "normal" sleep cycle... I just feel like hibernating, yep!