This is an old samurai technique of sleeping in focused attention.
Drop your attention into your Chi point (the chakra just below the navel). Maintain your attention at this location until you fall asleep. This is an excellent way to gather energy.
It is also very helpful for maintaining a hypnogogic state. If you still fall off to sleep too soon you will at least gather more energy for your next attempt. By using this method, I have been able to hover on that borderline between wakefulness and sleep for two to three hours; and with only three hours sleep still feel rested and alert in the morning.
An added advantage: it will be impossible for anyone, no matter how steathily they approach, to surprise you in your sleep. This was one of the reasons for it’s use by the warriors of old.