[Joeri]

I think a more important advice than follow the ultradian rhythm is simply to sleep enough. The negative effects on productivity of insufficient sleep are severe but hard to recognize. It's a widespread problem, half of everyone doesn't sleep enough.

I struggle with this myself. I've noticed that if i consciously sleep in excess of 8 hours a night every night my productivity jumps, but i always tend to drift back to 7 hours sleep.