Q: Can I not command my body to cut down its sleep to two or three hours only? I have heard that many Yogis are able to minimise their sleep.
A: It is not possible to do at once what you like with the body. If the body is told to sleep only 2 or 3 hours, it may follow if the will is strong enough but afterwards it may get exceedingly strained and even break down for want of needed rest. The yogis who minimise their sleep succeed only after a long tapasya in which they learn how to control the forces of Nature governing the body.
Ref: Guidance from Sri Aurobindo – Letters to a young disciple