Why Do We Need Sleep?
Exploring the Science Behind Sleep
Your internal "biological clock," managed by the circadian rhythm, plays a crucial role in regulating your sleep patterns, dictating when you experience fatigue and the urge to sleep, as well as when you feel refreshed and alert. This 24-hour cycle guides your daily transitions, with drowsiness gradually building up as the day progresses, culminating in the evening as you approach bedtime.
This phenomenon, also referred to as sleep-wake homeostasis, appears to be associated with adenosine, a natural compound produced in the brain. Adenosine levels steadily rise throughout the day as you become more fatigued, and they are then metabolized during sleep.
The circadian rhythm is also influenced by light. Within the brain, a specialized region of nerve cells called the hypothalamus, housing a cluster known as the suprachiasmatic nucleus, processes signals received when your eyes are exposed to either natural or artificial light. These signals aid the brain in distinguishing between day and night.
As natural light diminishes in the evening, your body releases melatonin, a hormone that induces drowsiness. Conversely, when the sun rises in the morning, cortisol, a hormone promoting energy and alertness, is released.
Phases of Sleep
Once we enter the realm of sleep, our bodies embark on a sleep cycle that can be divided into four distinct phases. The initial three stages are referred to as non-rapid eye movement (NREM) sleep, while the last stage is known as rapid eye movement (REM) sleep.
Stage 1 NREM: This initial stage signifies the transitional phase between wakefulness and slumber, characterized by a light sleep state. During this phase, muscle tension diminishes, and heart rate, respiration, and eye movements begin to decelerate, accompanied by a decrease in brain wave activity, which tends to be more active when we are awake. Stage 1 typically persists for a few minutes.
Stage 2 NREM: The second NREM sleep stage ushers in deeper sleep as heart rate and breathing rates continue their gradual slowdown, and muscle relaxation deepens. Eye movements cease, and body temperature drops. Although occasional bursts of heightened electrical brain activity occur, the overall pattern remains slow. Stage 2 is typically the lengthiest of the four sleep stages.
Stage 3 NREM: This stage assumes a crucial role in ensuring you wake up feeling rejuvenated and alert the following day. Heart rate, respiration, and brainwave activity all reach their lowest points, while muscles are as relaxed as they will be during sleep. Initially, this stage is lengthier and gradually decreases in duration throughout the night.
REM: The first episode of REM sleep emerges around 90 minutes after falling asleep. As the name suggests, your eyes dart back and forth swiftly beneath your eyelids. Breathing rate, heart rate, and blood pressure start to rise, and this is typically when dreaming occurs. During REM sleep, your limbs become temporarily paralyzed, a mechanism believed to prevent you from physically acting out your dreams. The length of each REM sleep cycle extends as the night progresses. Numerous studies have associated REM sleep with memory consolidation, the process of transforming recent experiences into long-term memories. The duration of REM sleep decreases with age, causing individuals to spend more time in NREM stages.
These four stages repeat in a cyclical fashion throughout the night until you awaken. Typically, each cycle spans about 90-120 minutes for most individuals. NREM sleep constitutes approximately 75% to 80% of each cycle. Occasionally, you may briefly wake during the night but not retain any recollection of these episodes, which are referred to as "W" stages.
