Throughout the day, downward pressure is exerted on the spine due to gravity and your vertebrae compress. As a result, fluid drains out from between your vertebral discs and you actually shrink up to 1 centimeter (0.4 in) by the end of the day. Similarly, when your back is relieved of all that stress at night, the fluid is allowed back into the joint connections, permitting the body to stretch an extra 1 centimeter (0.4 in) or so.
Although this height difference is not that significant, the lack of pressure also enables children and adolescents to grow while sleeping. In fact, we can only grow while asleep. This is because of both the pressure that is taken off the spine and legs when lying down and the growth hormones that are released while sleeping.
Although not everyone sleepwalks, enough of the general population (around 30 percent) has sleepwalked at least once in their lives to include it on this list. Also called somnambulism, sleepwalking is technically a sleep disorder in which the brain is in a semiconscious state, performing complex tasks such as getting out of bed, going to the kitchen, even driving.
Obviously, this can be very dangerous. But sleepwalking is relatively common, especially among children. Parents, roommates, and friends often report that the somnambulist in question will act dazed and confused while performing bizarre behaviors such as preparing a meal, only to return to bed.
Scientists still are not sure why people sleepwalk, although research has shown that it may be genetic. Sleepwalking usually occurs during slow-wave sleep, in which the brain is busy processing the day’s memories.
This may explain why a sleepwalker’s short-term memory is not very active while in this trancelike state. In fact, the person in question will have no memory whatsoever of the previous night’s events when they wake up in the morning.
5. Body Spasms
When you fall asleep, your body jerks. Every time. As described above, we are usually paralyzed while asleep to protect us from acting out our dreams. However, there is a gray area, a moment when the body is not physically asleep but not awake, either.
This is when most people experience what is called a hypnic jerk. It is believed to be a delay between the brain sending the message to relax and the nervous system getting this message.
We do not fully understand how this reaction came to be. Some scientists suggest that it is left over from a primitive reflex that misinterprets falling asleep to falling out of a tree. Others say that it is just the nerves “misfiring” as they are turned off.
Whatever the cause, hypnic jerks are one of the few reminders of the complex processes that occur while asleep that we can actually observe while awake. This is because a hypnic jerk upon falling asleep can be so violent in some cases that it actually wakes a person back up.