To understand the nature of sleep and its stages, it’s important to understand the difference between sleep and wakefulness. Being awake is a state of consciousness accompanied by heightened perception, realistic thinking, environmental responsiveness and physical activity. In contrast, sleep is a behavioral state of decreased perception, relatively low responsiveness to the environment and physical inactivity (or rest).
A quiescent body is the most obvious characteristic of sleep, but the brain remains active at varying levels while regulating sleep and performing vital tasks. Indeed, during certain phases of sleep, the brain is more active than during wakefulness.
There are two fundamental sleep states: REM (rapid eye movement) sleep and non-REM sleep. Each is regulated by a different part of the brain; and the difference between those states is as profound as the difference between sleep and wakefulness. REM sleep is regulated from the brainstem, whereas non-REM sleep is regulated from higher brain centers.
As a practical matter, scientists differentiate between the three states (wakefulness, non-REM sleep and REM sleep) by measuring
- electrical activity in the brain via electroencephalography (EEG)
- eye movements via electrooculography (EOG)
- muscle activity via electromyography (EMG)
The following are more precise descriptions of the behavioral and physiological changes across these states.
A waking state may be quiet, physically active or mental active.
Non-REM sleep has three stages (1, 2 and 3) which are differentiated by their degrees of sensory and motor disconnects from the environment. The magnitude of those disconnects is otherwise known as sleep depth. (Stage 3 is the deepest.) Increasing depth means
- slowing, mostly homogenous electrical activity in the brain
- decreased slow-rolling eye movements (ceasing completely in stages 2 and 3)
- decreased muscle activity (though movement is always possible)
- decreased respiration and heart rate, but increasing regularity
- decreased oxygen consumption
- growth hormone secretion (stage 3)
- decreased sensitivity of pain receptors
- stable body temperature control
- realistic and analytical thought processes (similar to wakefulness)
REM sleep is characterized by varying levels of sensory disconnection from the environment and an inability to move. The important characteristics are
- rapid and heterogeneous electrical activity within the brain (similar to that of wakefulness)
- inhibition of voluntary muscles leading to near-paralysis with some irregular twitches
- rapid eye movements similar to those of wakefulness
- decreased control of body temperature
- faster and irregular heart rate and respiration (relative to non-REM sleep)
- penile tumescence
- increased oxygen consumption
- hyper-associative dreaming with bizarre thoughts
Although the three states (wakefulness, non-REM sleep and REM sleep) have very distinct physiological characteristics, there may be overlap under certain circumstances. For example, sleepwalking occurs mostly during non-REM deep sleep and presents a behavioral characteristic that is normally associated with wakefulness.
Every human is in one of these three states at any given time. On average, adults are awake for 2/3 of the 24-hour day and asleep for the other 1/3. When sleeping, the relative proportions of sleep states are roughly
- 50% in light non-REM sleep
- 20-25% in deep non-REM sleep
- 20-25% in REM sleep
An amazing statistic: In a 90-year lifespan, a person will probably have slept for 30 years and will have spent 10 years dreaming!
How Will SleepRate Help You?
Use SleepRate for a few days to identify sleep-wake patterns and to measure sleep duration, efficiency and structure. If those measurements suggest a condition that warrants further investigation, you’ll be prompted to consult a physician for an in-depth sleep evaluation. Otherwise, SleepRate will provide you with personalized advice on how to improve your sleep.