It's a crazy old world! Sleep is vital, and especially to poker, and I do at times find myself dozing off while sitting playing, so the best solution is to mobile up and go walkabout, while still playing. Further proof of this is that last night, for once I didn't fancy playing poker, so ended up in a 3 hour session playing FIFA15. I didn't fall asleep once! I know that if I had been playing poker I would have dozed off.
So, why is this? It's simple really. Being active keeps you awake longer than sitting at a table playing poker. Also, physical fitness impacts on your brains ability to work cerebrally for longer. So, get off your assess and walk around. When playing poker, and when not, do some exercise, you can play longer and stay alert longer. This is why physical fitness is key to playing sit down poker!