For one thing, sleep is not nearly as vulnerable a state as it appears. Sleepers are highly sensitive to some sounds, like a baby’s whine or an unusual thump or voice. And as Dr. Siegel put it, sleepers are less vulnerable to harm than they would be if they were out on the street late at night.

For another thing, the new paper argues, evidence from other animals strongly suggests that the need for sleep drops sharply during the most important waking hours. Migrating killer whales are alert and swimming for weeks on end, and seemingly just as alert as when well rested, studies find. Recent research suggests that the same goes for white-crowned sparrows : they get far less sleep than usual when migrating.

Consider the big brown bat, perhaps the longest-sleeping mammal of them all. It snoozes 20 hours a day, and spends the other 4 hunting mosquitoes and moths in the dusk and early evening. “Increased waking time would seem to be highly maladaptive for this animal, since it would expend energy and be exposed to predatory birds with better vision and better flight abilities,” Dr. Siegel writes.

In humans, it is well known that sleep quality changes with age, from the long, deep plunges of early childhood to the much lighter, more frequently interrupted five or six hours that many elderly people call a night’s sleep. Doctors have long debated whether elderly people are sleep-deprived as a result, or simply need less restful slumber. In Dr. Siegel’s view, it’s a matter of tradeoffs: older people no longer have a child’s need to grow, which requires deep, long sleep and may have more need and more ability to do things for themselves instead.

In short, when there is hay to be made, animals tend to make it, whether the sun is shining or not. Depending on the animal, a long period of waking may or may not be followed by a long recovery sleep.

The theory also supports what people already suspect about early birds and night owls: they are most alert when they are naturally most productive. And they can feel strung out if their work schedule doesn’t match.

None of which is to say that good sleep is unnecessary or that serious sleep problems do not exist. It is and they do. But the theory does suggest that a stretch of insomnia may not be evidence of a disorder. If sleep has evolved as the ultimate time manager, then being wired at 2 a.m. may mean there is valuable work to be done. Time to turn off the “South Park” reruns and start doing it.