I sleep for 4-5 hours, get up for about an hour or so, and then try to sleep another 3-4 hours. Previously, I'd wake up after 4-5 hours and spend 2-3 hours trying to fall asleep again. An hour of activity during the night seem to be much more efficient as I get more sleep in total, and obviously feel rested afterwards.

Occasionally, I've stayed up, and while I've usually felt quite well for several hours without noticeable decreases in my cognitive abilities, I've been completely exhausted by lunchtime instead.

Perhaps it is an age thing?