A good rule of thumb is every couple of hours take fifteen minutes off. Naps are also very helpful, short naps. Even a ten or fifteen minute nap in the middle of the day can be the equivalent of an hour and a half of extra sleep the night before, and it can raise your effective IQ by ten points. Daniel Levitin
Short naps are good. Given modern workplace demands, this is not possible for many people - but if you have the option, try napping for ten to twenty minutes in the afternoon, preferably lying down in a darkened room. Andrew Weil
A good rule of thumb is every couple of hours take fifteen minutes off. Naps are also very helpful, short naps. Daniel Levitin