Sleepwalking, Sleep Terrors, Sexsomnia, and Human Brain Evolution

It can be hard to get a good night’s sleep if you suffer from a sleep disorder. It can also be hard to get a good night’s sleep if you are sleeping next to someone with a sleep disorder. Currently, little is known about the origin or neurobiology of sleeping disorders. Recent research has revealed that sleepwalking, night terrors, and sexsomnia may be due to an underlying pattern of brain activity shared by dolphins and aquatic seabirds.

Sleepwalking is more than just getting out of bed and walking around. Sleepwalkers often perform many different normal, routine behaviors. Rarely, sleepwalkers may climb out a window or even attempt to drive a car. During the daytime, sleepwalkers are at a greater than average risk of regular headaches or migraines. Older people who sleepwalk report more daytime depression. Young people, especially males age 20 to 29 who use marijuana, are also more likely to sleepwalk. The mechanisms underlying these features of sleepwalking are unknown.

Sleep terrors are characterized by the onset of abrupt terror associated with a frightening scream, usually shortly after going to bed. The person physically experiences the terror and displays signs of intense arousal including dilated pupils, fast heart rate and breathing, as well as profuse sweating. While sleepwalkers are often quiet, people with sleep terrors will open their eyes, raise up their heads, and start screaming; they may even jump out of bed to alert others.

Sexsomnia is the display of sexual behaviors while asleep. Patients may masturbate or make loud sexual vocalizations while asleep. Some people, mostly women, report having spontaneous sleep orgasms that are not accompanied by erotic dreams. Bedpartners report being sexually molested to the point of forced sexual intercourse. The perpetrator usually has total amnesia upon awakening (although this claim has been challenged in recent literature). Interestingly, sexsomnia is not associated with unusually high sex drive during wakefulness.

The relatedness of these disorders is demonstrated by the fact that about 10 percent of people with classical sleepwalking or sleep terrors display sexsomnia during sleep. Sleepwalking and sleep terrors equally affect adult men and women. In contrast, sexsomnia is reported more often in men.

Sleepwalking, night terrors, and sexsomnia are considered to occur during non-rapid eye movement sleep, a phase of sleep typically not associated with dreaming. Dreams that occur during non-rapid eye movement sleep tend to lack a narrative. During the past few years, a series of studies involving electroencephalography and functional brain imaging have shown that the brain regions responsible for the control of movement and emotions (known as the limbic system) show surprisingly large levels of activation as compared to other brain regions. This is surprising because control subjects show quite low levels of activation in these two brain regions. (To learn more about the limbic system and how the brain controls sleep, see my book.)

Scientists speculate that the front-to-back gradient of localized brain activity seen in people with sleep disorders might be normal during early brain development and then convert into an adult pattern with maturation. The coexistence of awake-like and sleep-like activity patterns may be normal for the developing brain and may recapitulate a pattern of brain activity shared with other vertebrates.

A similar coexistence of awake-like and sleep-like patterns of brain activity occurs in sea-going birds and aquatic mammals, such as dolphins. This well-known pattern of brain activity allows these animals to continue swimming, flying, or monitoring for predators. The only difference is that in the bird and dolphin brains the activity gradient is lateralized to one hemisphere at a time while in humans the disordered sleeping brain has an anterior-to-posterior gradient of activity during non-rapid eye movement sleep. It is possible that the peculiar features of these human sleep disorders are revealing hidden secrets about how the human brain evolved.