I learned from that:
The optimal nap time is 7-8 hours after wake up time (early / mid-afternoon depending on when you wake up), for 20-45 min (to prevent sleep onset issues). Longer naps should be taken earlier in the day if you are trying to catch up on sleep debt.How to nap – Boston.com
Here is the relevant passage from his book on sleep debt:
When you don’t get enough sleep, one consequence among many is that adenosine concentrations remain too high. Like an outstanding debt on a loan, come the morning, some quantity of yesterday’s adenosine remains. You then carry that outstanding sleepiness balance throughout the following day. Also like a loan arrears, this sleep debt will continue to accumulate. You cannot hide from it. The debt will roll over into the next payment cycle, and the next, and the next, producing a condition of prolonged, chronic sleep deprivation from one day to another. This outstanding sleep obligation results in a feeling of chronic fatigue, manifesting in many forms of mental and physical ailments that are now rife through industrialized nations.