Why Do We Sleep?

    

      Catch 40 winks, nod off, hit the hay we all sleep, spending roughly one-third of our lives doing it. Why humans need to sleep, though, is a question scientists still haven’t answered.

   What’s obvious is that without sleep, we lack energy and our thinking process can become muddled. Sleep deprivation can also lead to accidents on the road or at work, various health ailments, decreased  sex drive, and symptoms of depression, among other problems. And while reported cases of human beings directly dying from lack of sleep are rare, the physiological changes that occur from sleep deprivation can be more detrimental, and possibly fatal, than going without food.

   Over the years, scientists have advanced several theories about the role sleep plays in human health. One theory suggests that sleep, and the conservation of energy that goes with it, helped humans and other species evolve. Using less energy for part of the day lowers the demand for food. For humans, sleeping at night meant they were conserving energy during the time when it would be hardest to find food. Some scientists see a link between this theory and one called the adaptive or evolutionary theory. Early humans saw the value of staying inactive at night in order to avoid drawing the attention of nocturnal predators. This prolonged inactivity evolved into sleep.

  Another explanation for why we sleep is  the restorative theory. During sleep, parts of the body restore themselves—tissues are repaired, hormones are released, proteins in brain cells are synthesized. Neuroscientists talk about the brain’s plasticity—its ability to modify its internal structure as it encounters changes in the environment or the body itself. Sleep seems to play a role in this plasticity, as neurons forge new pathways during those hours, especially in young people. While asleep, the brain also processes memories so that they can be drawn upon for future use. Research suggests that the neural connections that create memories strengthen when we sleep.

   A more recent theory about the importance of sleep has called it a biological dishwasher.” During sleep, the brain flushes out waste products that accumulate there during the day. One of these substances is adenosine, which is found in all cells. In the brain it’s created during neural activity, and as it accumulates it makes us feel sleepy. When  we actually do sleep, the body flushes the adenosine out of the brain, helping us feel revived when we wake. In 2013, this idea of sleep and biological cleansing received a boost from research done on mice. Their brain cells shrank while they slept, creating pathways for spinal fluid to pass through. The fluid flowed ten times faster during sleep than when the mice were awake. The flow of the spinal fluid helped carry away the brain’s waste products as well as proteins that can harm the brain when too many of them accumulate there.

    Today, scientists don’t agree that any one theory explains why we sleep. They continue to probe what exactly happens in the brain when we grab some shut-eye.