Stages of Sleep
Up until the 1950s, before we understood the various stages of sleep, most people
thought sleep was a time when our brains and our bodies remained relatively dormant.
Scientists and researchers have since revealed that, for our brains at least, sleep
is quite the contrary—a very active state affecting both our physical and mental
health.
During sleep, while our bodies rest and recharge, we cycle between two main types
of sleep: rapid eye movement sleep (REM)
and non-rapid eye movement sleep (NREM).
These two types of sleep usually take place over four non-REM stages and one REM stage throughout the night.
NREM accounts for longer periods of sleep during which our brain activity and bodily
functions slow down. During NREM sleep, our bodies repair and regenerate tissue,
build bone and muscle, and are thought to recharge our immune systems. REM is defined
by brief spurts of increased activity in the brain. REM sleep is considered
the dreaming stage of sleep.
Sleep Stage 1—NREM Sleep
During this stage, our sleep is quite light and we can be awakened fairly easily.
Our eye movement and muscle activity also slow down.
Sleep Stage 2—NREM Sleep
This is also a stage of light sleep, but our eye movement stops and our brain waves
slow down. Our heart rate also begins to slow and our body temperature decreases.
This is when our bodies prepare to move into deep sleep.
Sleep Stages 3 and 4—NREM Sleep
These sleep stages are also known as "deep (slow-wave) sleep," the most restful kind of sleep.
During these stages, extremely slow brain waves, called delta waves, are produced
and there is no eye movement or muscle activity.
REM Sleep
During this final stage of sleep, we switch into REM mode. REM sleep typically occurs
about 70 to 90 minutes after we fall asleep. Our breathing becomes shallow and irregular,
our muscles relax, our eyes begin to move rapidly, our heart rate increases, and
our blood pressure rises. During this stage we often experience vivid dreams.
Healthy sleep
is characterized by this specific "sleep architecture,"
or sequence of stages. The sleep cycle usually begins with a period of about 80
minutes of NREM sleep followed by about 10 minutes of REM sleep. This 90-minute
cycle is repeated four to six times each night. If the sequence is interrupted (for
example, by external noise or a sleep disorder), the quality of our sleep suffers.
Next: Healthy Sleep