It's highly variable for me and I suspect it's the same for most people even if they don't realize it. The outcome is subject to so many variables - what you ate, the physical activity you underwent, any events/stresses of the day, what else you may have ingested, drugs, caffeine, alcohol, whatever - the list goes on and on. Under ideal circumstances, I'm OK with >6 hours. I feel like 7 is the best general number for most scenarios. It's rare that I can get a good night uninterrupted sleep for more than 6-7 hours. If I get less than 6 hours, it's rare that I will feel good about it. Drag my feet, groggy and irritable, feel like a bag of shit if I've been drinking and don't get much sleep. Most of the time I will feel pretty good towards the second half of the day, mostly recovered. Unless I was a real idiot the night before, or if I really just barely slept. Today is a good example actually... I drank a little too much last night, only slept 5ish hours, beginning of the day sucked. But I still managed to go for a jog this evening, and I did better than all of my previous jogs over the past few weeks (just started exercising again); so I couldn't have been *that* fucked over by the lack of sleep. I feel like tonight will be a good night's sleep though!