Sleep is a physical and mental resting state in which a person becomes inactive, as well as unaware of their environment and external stimuli. In normal sleep the body’s temperature lowers, blood pressure decreases, heart and breathing rates slow, as do almost all other bodily functions. However, the brain is as active while you sleep as it is while you are awake.
Normal sleep is also characterized by two separate states relating to REM (rapid eye movement); non-REM sleep and REM sleep. These two states alternate in approximately 90-minute cycles, with 4-5 of these cycles occurring in any 8-hour period of sleep.
Non-REM-sleep cycles through four stages within each cycle. As a person drifts asleep, stage one begins. Less than 5% of non-REM sleep is spent in stage one. Muscles and breathing begin to relax and the sleeper can be awoken very easily. Stage two begins shortly after the onset of stage one and is considered the official beginning of sleep. Around 45% of non-REM sleep is spent here and it is characterized by lack of eye movement and increased brain activity.
As sleep progresses and becomes deeper, the sleeping individual becomes more difficult to wake. This is the onset of stage three non-REM sleep. Breathing, heart rate, blood pressure and muscle activity have lowered. About 12% of non-REM sleep is spent here and leads into stage four, which is considered very deep sleep. 14% of non-REM sleep is spent in stage four. People who are woken during this stage are often groggy and disoriented for several minutes.
The remaining approximately 20-25% of the sleep cycle is spent in REM sleep. REM is separated from non-REM sleep by eye movement, irregular breathing, heart rate and blood pressure. Also, the brain during REM sleep is nearly as active as in an alert or wakeful state. Dreaming occurs during REM sleep and the brain blocks signals to the muscles so the sleeper remains immobile as not to “act out” dreams.
Why do we need sleep? We can’t be sure of all the reasons why we need sleep, but there are two common theories for it function, Restorative and Adaptive.
Restorative refers to sleep enabling the mind and body to repair and re-energized. While we sleep the brain is performing “housekeeping”. It is suspected that sleep allows long term memory to be organized, new information to be integrated, repairing and renewing tissues and nerve cells, as well as restoring normal levels of chemicals throughout our bodies. These functions play a vital role in the performance of our health and wake time.
Adaptive refers to sleep having evolved to accommodate an organism’s living environment and food source. A good example is hibernation. Animals living in a cold climate where food is scarce become dormant for this period in order to conserve energy and resources. Prior to our modern lives, humans spent a lot of time hunting and finding food. This is much easier to do in the day time. Humans also spent time hiding from predators; this is easier to do at night. As a result humans have adapted to sleeping 6-8 hours during nightfall.
Another example of adaptive sleep can be seen when comparing carnivores sleep patterns to that of herbivores. Herbivores need to consume large quantities of vegetation to sustain life and therefore sleep little in order to accomplish this. While carnivores generally will hunt prey where one feeding can sustain them for days, therefore spending the rest of their time sleeping to conserve energy until the next hunt. Omnivores usually fall somewhere in the middle.
It has been argued that adaptive sleep is the more crucial of the two theories, claiming that the restorative functions occurring during sleep in humans, do so only because it happens to be convenient. While in other species with different sleep patterns, sleep serves different functions across various species.
Regardless of the theories, sleep for humans is not only important, it is a necessity! Sleep is essential to a person’s physical and emotional well being. Studies prove that without proper sleep, a person’s ability to perform even basic tasks declines dramatically. Furthermore, being deprived of sleep can cause irritability, lack of concentration and a loss of focus. Due to sleep’s restorative nature, a weakened immune system is also common with a lack of sleep.
How much sleep do we need? Experts on sleep agree that anywhere from 6-8 hours of sleep per night is sufficient. Believe it, that’s one third of your life! But, like all circumstances, it comes down to the individual. Some people are capable and happy with just five hours of sleep while others need 8 or more hours per night. The best way to determine your sleep requirement is to fall asleep without an alarm clock. Your time spent sleeping and then naturally arousing will best fit your personal need for sleep.