Sleep and Sleep Measurement

All animals exhibit some form of sleep. This can range from the sleep-like state seen in nematodes to sleep in dolphins where one-half of the brain sleeps at a time. Sleep keeps one functioning at their best physical and mental health. Without the proper amount of sleep, memory, hand-eye coordination, blood pressure, and eating habits can be negatively affected.

Sleep is defined as a period where activity is reduced and the response to external stimuli is diminished. The sleep state is reversible (unlike coma or hibernation), and associated with a normal posture. For humans, that posture is typically lying down with eyes closed. Posture can, however, vary across species. Sleep was thought to be a passive process wherein the brain is at rest, however, scientists today know that the brain is highly active during sleep working to recuperate, regenerate, release growth hormones, and remove cellular debris. These processes are observed while studying the brain waves and other physiological properties of a sleeping person.

Sleep is measured using polysomnography. Polysomnography is a combined measurement of brain waves (electroencephalography EEG), muscle activity (electromyography - EMG), heart rate (electrocardiography - EKG), eye movements (electrooculography - EOG), body temperature, and breathing. By placing electrodes on various regions of a person's scalp overall brain activity and muscle tone can be measured and recorded over time. Over the course of a night's sleep, there are characteristic changes that occur in the different stages of sleep.

There are five stages of sleep. These are characterized as rapid eye movement sleep (REM) or N-REM (stages 1-4, non-REM stages). As the stages progress to deeper sleep, the waves become lower in frequency (more spread out) and higher in amplitude (height).

• Stage 1 This stage occurs as one is starting to fall asleep, laying in bed with eyes closed, but not quite asleep. The brain waves have slightly slowed from the awake (alpha waves) to theta waves.
• Stage 2 In this stage sleep has begun and slower waves begin to form. It is still light sleep, but characteristic features such sleep spindles (bracket) and k-complexes (peak at the end of stage 2) start to appear. The spindles and k-complexes are thought to be involved with maintaining sleep when, for example, there is a sound or other outside influence.
• Stage 3/4 These are the deepest stages of sleep, characterized by slower waves called delta waves. During these stages, the body is recuperating and regenerating.
• Rapid Eye Movement (REM) This stage is often called the paradoxical stage of sleep when a person is typically dreaming, and the brain is very active. The EEG will show a fast-desynchronized pattern similar to someone that is awake, but the EMG will show decreased muscle tone/body paralysis, and unlike the other sleep stages, the eyes will move back and forth.

Over the course of a healthy night's sleep, the brain will oscillate between 4-5 cycles of N-REM and REM sleep in the manner above. The deepest stages of sleep occur during the first few cycles at the beginning of the night, and lighter stages predominate in the next cycles in the second half of the evening. It is easier to be awoken during the lighter stages 1, stage 2, and REM, and more difficult to be awoken during stage 3 or 4. If woken up during the deeper stages, one will feel very groggy. There are unique alarm clocks to predict when a sleep cycle will end and wake someone up at the ideal time feeling more rested. While this represents a perfect night's sleep, there are many disorders where this does not occur.

When sleep is not perfect, a variety of problems can be present. Doctors will perform a sleep study and observe a patient sleeping overnight. The observation will include a polysomnography. A few of the most common sleep disorders include insomnia, bruxism, and sleep apnea. Insomnia occurs when a person is not getting enough sleep and can include an inability to fall or stay asleep throughout the night.

A polysomnograph reading may show a person waking up more often throughout the night; this could be due to other problems such as bruxism or sleep apnea. Bruxism is the grinding of teeth while sleeping; the EMG would show bursts of activity during grinding. Sleep apnea is disordered breathing throughout the night. The patient will wake up multiple times per night when breathing is blocked or stopped. Once the doctor identifies the problem treatment can begin.

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