If you’re interested in sleep research, The Promise of Sleep by the pioneer of sleep research Dr. Dement is an excellent read.
The optimal times to nap are after lunch and during mid-afternoon. During those times of the day, the body’s circadian clock puts the body at its lowest points of alertness, which translates into higher efficiency in falling asleep.
As to how long to sleep, Dement’s research shows that the beneficial effects vary with the amount of sleep debt you’re carrying around. One of the most interesting takeaways from the experiments and studies in his book is that sleep debt builds up linearly and can stick around for at least two weeks. This means that a) if you didn’t sleep your 8 hours a week ago, you can still be carrying the debt around and have lower performance, and b) you need to sleep one hour for each hour that you missed in order to lower your sleep debt. That suggests that as long as your body is missing sleep, napping for longer periods of time will more strongly decrease your sleep debt and increase your reaction time and alertness.
Author: Edmond Lau, Author of The Effective Engineer