As an animal species, human being need to sleep about 1/3 of every day. Sleep deprevation lowers productivity asmong hourly-paid workers. Recently, I heard of a large scale experiemnt on public radio concerning:

1. Sleept efficiency - the % of time you actually sleep while in bed

2. Does naps actually imporve productivity? Naps are better than more sleeping hours at night?

3. Other information concerning sleep, nap, and productivity

You can find more information on 

1. Everyone enjoys a nap https://freakonomics.com/podcast/nap-time-for-everyone-ep-468/

2. google "Freakonomimcs on sleep"

The most important advice or takeaway for us seniors and people engaged in creative work is "TAKE MORE NAPS"