I once had 3-4 hours of sleep per night and became an irritable wanker and felt like crap. I have at least six hours on most nights now and I'm perhaps still an irritable wanker on occasion, but I do feel a lot better. In the weekend I'll try to have more sleep but it doesn't always work out that way. The best night for sleep is Friday night for me, as it's when I've just gone through five days of long work and little sleep and I often go to bed early and wake up later than normal. For some reason, I rarely get a lot of sleep on a Saturday night... possibly because of all that sleep on Friday night.