As a firefighter/EMT I live by the adage "eat til you get tired, then sleep til you get hungry." Kidding aside, as a soldier I learned to sleep whenever you have down time. In a survival situation that may be when you are safe and secure and just waiting for rescue, or when it is too dark to do anything useful or you are weathered-in.
The military and fire/ems world has studied the subject for years. As little as 15 minutes of sleep at a time is helpful but as Montanero intimated, you have to crash eventualy. In Iraq I was fortunate to get a 4 hour and a two hour sleep per day and then get a day once a week to sleep as much as I wanted, usualy 16 hours in two or three segments over 24 or 36 hours.
As a medic in a war zone I found sleep disturbances
to be one of the most common complaints among soldiers. A disaster situation will have a similar effect. You can go a long time on little sleep but judgement is impaired. If you can then "sleep on it" before making major discisions.
When it is safe to sleep then Benadryl or other OTC sleep aids may be helpful. When sleep won't come then simply shutting down and going to your happy place is better than nothing. Ear plugs and a sleep mask or anything to darken your eyes works for me. No heavy meals or heavy excercise within 2 or 3 hours of sleep time. Some people can't use caffiene within 6 hours of sleep time.
One more thing. I can't sleep when I'm freezing so I find myself feeding a fire most of the night and then sleeping in the sun like a turtle on a log during mid day.
leo