Physiology and Stages of Sleep

Introduction

Sleep is a very complex activity that is more than just closing the eyelids. Researchers believe that human beings spend about one-third of their entire life sleeping. It is a state of unconsciousness in which the brain is in a state of rest. During sleep, the brain is more reactive to internal stimuli than external stimuli. Internal stimuli are the changes or feelings that occur within a person inside the body. A few examples of internal stimuli are thirst, hunger, and change in a person’s emotions. External stimuli are the changes that happen outside the body and are passed on to the person through their senses. A few examples of external stimuli are touch, pain, vision, smell, and taste.

What Is Sleep?

Sleep is a complex biological process (processes that are very important for a person to live a healthy life) in which a person is in a temporary state of rest, becomes physically inactive, and is not aware of the surroundings. Researchers suggest that an adult requires an average sleep of seven to eight hours per day, whereas infants need about 16 to 18 hours of sleep a day. Sleep is one of the main factors for various functions of the brain, including the communication of brain cells with each other. Even though a person is unconscious during sleep, it is different from other forms of unconsciousness like coma or fainting. This is because the duration and the cycle of sleeping are predictable, and the unresponsiveness of a person during sleep can be reversed by just an external stimulus like touch.

What Are the Factors That Regulate Sleep?

According to researchers, two main factors regulate sleep. They are as follows:

• Circadian Rhythms: These are mental, behavioral, and physical changes that undergo in the body of a living organism in the twenty-four hours cycle. These are controlled by a small region present in the center of the brain and are affected by changes in the environment, like light and darkness. Hence, it is responsible for the act of sleeping at night (dark) and waking up during the daytime (light).

• Sleep Drive: This is the urge demanded by a person’s body to sleep, which is similar to the urge to eat when a person feels hungry. If a person feels exhausted, their body forces them to fall asleep. Even if the person controls or prolongs it, after a certain point, the person is driven to fall asleep. In case the person is not able to fall asleep due to any reason, the body has the power to engage in microsleep with eyes open, which lasts for just a few seconds.

What Is the Physiology of Sleep?

Neurons are messengers that are present in the brain. They use signals to transfer information between different regions within the brain and between the brain and other parts of the nervous system (consisting of the brain, spinal cord, and a complex group of nerves). Neurotransmitters are chemicals that help in the communication of nerves. The neurons responsible for sleep and staying awake are as follows:

• Neurons located in the brain stem (the junction at which the brain and spinal cord meet) produce neurotransmitters (chemicals) that keep the brain active while a person is awake.

• Neurons present at the base of the brain are responsible for making a person sleep. These neurons cut off the signals that keep a person awake.

The physiological processes that happen in the body system during sleep are as follows:

• Cardiovascular System (Consists of the Heart and Blood Vessels): Changes in heart rate and blood pressure take place during sleep. Normally, an increase in the heart rate and blood pressure is seen in the morning when a person is going to wake up from sleep. This indicates a high risk of a heart attack during that period.

• Respiratory System (Group of Organs That Helps in Breathing): There are changes in the respiratory flow of air during sleep. There is a decrease in oxygen content and an increase in the carbon dioxide content in the blood when a person starts to fall asleep. The urge to cough due to the irritants present in the airway is very less during sleep.

• Blood Flow in the Brain: The rate of blood flow is less during the starting stage of sleep and increases progressively as the person falls into a deep sleep.

• Renal (Related to Kidneys): There is a reduction in the secretion of calcium, potassium, sodium, and chloride, which in turn reduces the urine flow during sleep.

• Endocrine (Related to Organs That Secrete Hormones Into the Blood Directly): Secretion of growth hormone takes place during the first few hours of sleep. A hormone called melatonin which causes sleepiness in a person is secreted in high amounts during the dark and reduced during the day time. The secretion of thyroid hormone also increases during late evenings.

What Are the Types of Sleep?

Throughout the sleep period, a person will cycle through two types of sleep. They are as follows:

• Non-rapid Eye Movement Sleep (Non-REM Sleep): This is the period from being awake to falling into a deep sleep. It consists of four stages.

• Rapid Eye Movement Sleep (REM Sleep): This is after one or one and a half hours after the person has fallen asleep. The person may have dreams during this phase.

What Are the Stages of Sleep?

A person may go through a sleep cycle four or five times on normal nights. The stages of sleep are as follows:

Non-rapid Eye Movement Sleep (Non-REM Sleep)

The stages of non-rapid eye movement sleep are as follows:

Stage One:

• It is the period between being awake and falling asleep.
• It lasts for about one to seven minutes and comprises two to five percent of the total sleep.
• Eyes remain closed, and eye movements slow down.
• The person may easily wake up from this stage if disturbed by noise.

Stage Two:

• It is the stage of light sleep.
• Heart rate, breathing, and body temperature fall.
• This stage continues for about 10 to 25 minutes and constitutes about 45 to 55 percent of total sleep.

Stage Three and Stage Four:

• It is the state of deep sleep.
• These two stages combined are known as slow-wave sleep (SWS).
• Stage three lasts for a few minutes and contributes about three to eight percent of sleep.
• Stage four constitutes about 10 to 15 percent of sleep and lasts for a duration of 20 to 40 minutes.
• There is no eye movement, and brain activity is decreased. It is hard to wake a person from this stage of sleep.

Rapid Eye Movement Sleep (REM Sleep):

• The sleep is not very deep.

• The brain activity increases and becomes similar to that of a person who is awake.

• The person’s body enters into a phase of paralysis, and the person may experience dreams.

• This stage lasts for about five to ten minutes in the first cycle of sleep. The duration increases as the sleep cycle progress.

Conclusion:

Sleep plays a vital role in each person’s life. It enables the body to rest and repair. Reduced sleep has been linked with conditions like high blood pressure, heart disease, poor mental health, diabetes (high blood sugar levels), and early death. Hence, it is important to get an adequate amount of sleep daily by following a healthy lifestyle and sleeping habits. People who experience trouble sleeping should seek the help of a doctor to get the right diagnosis and treatment.