What is a Normal Sleep Cycle?
Sufficient and restful sleep is a human need as basic as food, and is vital to our emotional and physical well being. Over the past few years, scientists have made great strides in identifying patterns and functions of brain activity in sleep. In order to understand insomnia, it’s also important to understand what is a normal sleep cycle.
The daily cycle of life, which includes sleeping and waking, is called a circadian rhythm, commonly referred to as the biologic clock. Circadian simply means “about a day”. Hundreds of bodily functions follow biologic clocks, but the most prominent of those are sleeping and waking.
Light signals that come through our eyes reset these circadian cycles each day. The response to light signals in the brain is an important key factor in sleep and in maintaining a normal circadian rhythm.
Light signals travel to a tiny cluster of nerves in the hypothalamus in the center of the brain, the body’s master clock, which is called the supra chiasmatic nucleus or SCN. This nerve cluster takes its name from its location, which is just above (supra) the optic chiasm. The optic chiasm is a major junction for nerves transmitting information about light from the eyes.
The approach of dusk each day prompts the SCN to signal the nearby pineal gland (named so because it resembles a pine-cone) to produce the hormone melatonin.
Experts agree that melatonin is an important hormone released in the brain that is critical for a normal sleep cycle. If we remain in darkness longer, more melatonin is secreted. When we get into bright light, levels drop. Research is ongoing to determine if high levels of melatonin cause sleep regardless of whether it is dark.
We are designed like most mammals to sleep at night and be awake during the day. There is also is a natural peak in sleepiness at mid-day, the traditional siesta time. Our sleeping and waking cycle is approximately 24 hours.
In sleep studies, subjects spend about one-third of their time asleep, suggesting that most people need about eight hours of sleep each day for a normal sleep cycle. Infants may sleep as many as 16 hours a day. Individual adults differ in the amount of sleep they need to feel well rested, however.
Daily rhythms inter mesh with a number of biologic and physical factors that may interfere or change individual patterns. For example, the firing of nerve cells in the brain may be faster or slower in different individuals. Such differences are fractions of a second but they can cause variations in the type, timing, and duration of a person’s sleep.
In women, their monthly menstrual cycle can shift the sleep/wake pattern. Changes in season or various exposures to light and dark often unsettle the sleeping pattern.
We need sunlight
Sunlight is important for circadian rhythms. People who are totally blind often have more problems with sleeping and other circadian rhythms. This is because their body can’t see the various periods of light and darkness.
Sleep consists of two distinct states that alternate in cycles and reflects differing levels of brain nerve cell activity. During a normal night’s sleep, we will progress through these stages about five or six times:
Non-Rapid Eye Movement Sleep (Non-REM) sleep is also termed quiet sleep. We can subdivide non-REM sleep into three stages of progression:
- First Stage (light sleep).
- Second Stage (so-called true sleep).
- Third Stage (deep “slow-wave” or delta sleep).
With each descending stage, awakening becomes more difficult. Nobody knows for sure what governs Non-REM sleep in the brain. A balance between certain hormones, particularly growth and stress hormones may be important for deep sleep.
Rapid Eye-Movement Sleep (REM) sleep is termed active sleep and most vivid dreams occur during this stage. REM-sleep brain activity is similar to that when we are awake, but our muscles are virtually paralyzed.
In fact, except for vital organs like lungs and heart, the only muscles not paralyzed during REM are the eye muscles. REM sleep may be critical for learning and for day-to-day mood regulation. Our brain has to work harder When we don’t get enough sleep.
The cycle between quiet (NREM) and active (REM) sleep generally follows the same pattern. After about 90 minutes of Non-REM sleep, eyes move rapidly behind closed lids, giving rise to REM sleep. As sleep progresses the Non-REM/REM cycle repeats. With each cycle, Non-REM sleep becomes progressively lighter, and REM sleep becomes progressively longer, lasting from a few minutes early in sleep to perhaps an hour at the end of the sleep episode.
Seems pretty simple and basic, doesn’t it? After all, we don’t have to learn how to sleep – it’s something we automatically know how to do, but some people still have problems. Why? Of course the whole purpose of this blog is to answer that nagging question and give you solutions to that you once again will be able to enjoy a normal sleep cycle.