Avian Migration: The Ultimate Red-Eye Flight
Birds that migrate at night enter a state of sleepless mania and gorge on foods by day, behaviors mediated by their biological clocks
Sleep(lessness) and Mania
If birds are migrating during the night and foraging during the day, when do they sleep? It’s unlikely that migrating birds get a normal amount of sleep, given the demands of long stretches of flying night after night. Migration is physically and cognitively demanding. Birds exert tremendous amounts of energy, navigating in the dark for thousands of miles, encountering new territories daily in which they must find food and avoid predators. Even if they managed to grab a catnap here and there, how do they function on such a sleep deficit? Our hypothetical businessman, churning out energy via stationary bike to power his red-eye flight before taxing his brain at work, would be severely cognitively handicapped after two days on this schedule. Humans lose one IQ point per hour of sleep deficit. But migratory birds, apparently, have developed a way to maximize performance while minimizing sleep.
How much, when and how they sleep remains unknown, but we are beginning to fill in knowledge gaps. Many birds can engage in unihemispheric sleep, where one half of the brain and body sleeps while the other half remains awake, so the animal can engage in at least some physical activity during sleep. Perhaps migrating birds employ this tactic on the wing; perhaps birds are able to find time to nap during the day; or perhaps migrating birds are unique among animals in their ability to go without sleep. There is clear evidence for the last in white-crowned sparrows (Zonotrichia leucophrys). In a University of Wisconsin study measuring sleep in captive birds, migrating white-crowned sparrows spent 63 percent less time sleeping than their nonmigrating counterparts. Furthermore, the structure of sleep was altered in migrating birds, which entered the rapid-eye movement (REM) stage of sleep more quickly than nonmigrating birds (see Figure 9, below right). Reduced REM sleep latency has also been observed in sleep-deprived humans, although the implications for cognition and performance are unclear. The migrating birds in this study did not compensate for reduced nighttime sleep by increasing their sleep intensity, nor did they sleep during the day. Yet remarkably, their performance on a cognitive test that assessed learning ability did not decrease. Humans with a similar degree of sleep deprivation perform very poorly on similar tasks, and the same was true for sparrows in nonmigrating condition. When nonmigrating birds were prevented from sleeping, their performance declined as expected. Yet somehow the same birds, when migrating, are resistant to the effects of sleep restriction; indeed, both their cognitive function and physical performance are in top shape during the migratory period. How birds accomplish this feat remains a mystery, but when solved, efforts to improve cognitive function in people that are sleep-deprived as part of their profession, such as soldiers and pilots, could be refined.
In the study described above, the authors found no evidence of daytime sleep or of unihemispheric sleep in the migratory birds. However, daytime unihemispheric sleep and micronaps, short sleep episodes lasting around 12 seconds, were observed in migrating Swainson’s thrushes (Catharus ustulatus), according to studies by Verner Bingman’s group at Bowling Green State University. Short, light bouts of sleep may allow birds some sleep recuperation without a significant loss of foraging time or risk for exposure to predators. Does this sleeping behavior occur in the wild? More work is needed to understand why species differ in sleep strategies during the migratory period.
Migrating birds seem to defy the “rules” of physiology. They become obese yet are elite endurance athletes; they hardly sleep, yet their brains and bodies are in top shape. Return to the image of taking the red-eye, night after night, running on next to no sleep and quick snacks. Now imagine that, instead of dreading this exhausting nightmare, you approach the task with unbounded energy. You feel no need for sleep. It’s difficult to sit still. You don’t need that triple latte; in fact you already feel as if you’re running on 1,500 milligrams of caffeine, or something stronger. If your spouse insists that you stay home and get some sleep, rather than taking that red-eye flight, you find that you can’t lie still in bed. You’re up, pacing the room with an irresistible urge to go somewhere. Most people would find that such sensations stretched their circadian inclinations. It may be more than coincidence that these actions are also characteristic of mania in people.
The hallmarks of mania include hyperactivity, reduced sleep, changes in sleep architecture, increased metabolism, increased goal-oriented behavior and increased stress hormone levels. Mania in people often occurs on a seasonal basis. Because these hallmarks are also characteristic of avian nocturnal migration, birds may be a useful research model for the development and treatment of seasonally occurring mood disorders in humans, such as bipolar disorder.