A nap of 60 minutes improves alertness for up to 10 hours. Research on pilots shows that a 26-minute "Nasa" nap in flight (while the plane is manned by a copilot) enhanced performance by 34% and overall alertness by 54%. One Harvard study published last year showed that a 45-minute nap improves learning and memory. Napping reduces stress and lowers the risk of heart attack and stroke, diabetes, and excessive weight gain.

Napping: the expert’s guide | Life and style | The Guardian