An insight to 'THE SLEEP CYCLE'

AN INSIGHT TO 'THE SLEEP CYCLE'Did you know, that sleep is considered as a more active state of the body than many of us think? So, what is the Sleep Cycle?

During sleep, our brain goes through various phases which is called a sleep cycle.

There are two basic types of sleep: rapid eye movement (REM) sleep and non-REM sleep (which has three different stages). In a typical night, a person goes through four to six sleep cycles. 

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Stage 1 (non-REM): is the switch from wakefulness to sleep. This relatively light sleep lasts for several minutes. Here your heartbeat, breathing, and eye movements slow, and your muscles relax. The brain waves begin to slow.

Stage 2 (non-REM): a phase of light sleep before the deeper sleep. Heartbeat/ breathing slow and muscles relax even further. The body’s temperature drops and eye movements stop.  Brain wave activity slows but there are brief bursts of electrical activity.  We spend most of our sleep here.

Stage 3 (non-REM): period of deep sleep that is needed to feel refreshed. Heartbeat and breathing are slowest. The muscles are very relaxed making it difficult to awaken us. Brain waves become significantly slower. 

REM sleep: occurs about 90 minutes after falling asleep. The eyes move rapidly from side to side behind closed eyelids. Breathing becomes faster and irregular, and heart rate and blood pressure increase to near waking levels.  Most dreams occur during REM sleep. 

The main purpose of sleep is to recover from the previous day’s tiredness, making a good quality sleep important for maintaining a good health. Without sleep we can’t form or maintain the pathways in our brain that lets us learn and create new memories, and it’s harder to concentrate and respond quickly.

Sleep is important to various brain functions including the communication of nerve cells and to help remove toxins in our brain that builds up while we are awake. It also affects the metabolism, immune function, mood, and disease resistance.  Research shows that chronic lack of sleep increases the risk of high blood pressure, cardiovascular disease, diabetes, depression and obesity.

It is normal for sleep cycles to change throughout the night. The first sleep cycle is the shortest, it last for 70-100 minutes, while later cycles are between 90 to 120 minutes.  

Anatomy of Sleep

Hypothalamus acts as a control center of sleep and arousal.  Suprachiasmatic nucleus (SCN) in the hypothalamus receive light exposure directly from the eyes and controls our behavioral rhythm. Damaged SCN causes erratic sleep throughout the day as they aren’t able to match their circadian rhythms with the light-dark cycle.  

Brain stem communicates with hypothalamus to control sleep cycle. Sleep-promoting cells reduce the activity of arousal centers in the hypothalamus and the brain stem.  Pons and medulla play a special role in REM sleep- it sends signals to relax muscles so that we don’t act out our dreams.

Thalamus is a relay center for information from the senses to cerebral cortex. During sleep, the thalamus is quiet detaching us from the external world.  But during REM sleep, the thalamus is active sending the cortex images, sounds, and other sensations that fill our dreams. 

Pineal gland receives signals from SCN and increases production of the hormone melatonin which helps put us to sleep. Scientists believe that peaks and valleys of melatonin are important for matching the body’s circadian rhythm to the external cycle of light and darkness.

Amygdala- involved in feeling emotions becomes increasingly active during REM sleep. 

Sleep mechanisms

Two internal biological mechanisms–circadian rhythm and homeostasis–work together to regulate when you are awake and sleep.  

Circadian rhythms control the timing of sleep called the body’s biological clock. Circadian rhythms synchronize with environmental cues (light, temperature) about the actual time of day, but they continue even in the absence of cues. 

Sleep-wake homeostasis reminds the body to sleep after a certain time and regulates sleep intensity.  This sleep drive gets stronger every hour you are awake and causes you to sleep longer and more deeply after a period of sleep deprivation.

Factors that influence your sleep-wake needs include medical conditions, medications, stress, sleep environment, and what you eat and drink. 

Night shift workers often have trouble falling asleep when they go to bed, and also have trouble staying awake at work because their natural circadian rhythm and sleep-wake cycle is disrupted.  In the case of jet lag, circadian rhythms become out of sync with the time of day when people fly to a different time zone, creating a mismatch between their internal clock and the actual clock. 

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