For many years, I've noticed that about a half hour after finishing moderately intense activity lasting more than 5 hrs (e.g. long bike ride, back country skiing, etc), I go out like a light. Nothing keeps me awake -- stimulants, self induced pain, freezing, whatever -- the sleepiness hits me like a wall so I stop wherever I am. After just 10 minutes of being out cold, I wake up feeling great/energized, the risk of going back to sleep drops to zero, and I'm good for hours.

I'm not fishing for medical advice as nothing is wrong, but I'm wondering what causes this? It can't be fatigue because if I ride/ski literally all day, I never get sleepy until about 1/2 hr after I stop. On the other hand, it is related to the amount of time I'm out since I never see this effect if I'm only out a couple hours. What gives?