The sleep cycle in humans is a complex process, governed by various physiological and hormonal factors. It's typically divided into two broad types of sleep: Rapid Eye Movement (REM) sleep and Non-Rapid Eye Movement (NREM) sleep. Each sleep cycle, consisting of both REM and NREM stages, lasts about 90 to 110 minutes and is repeated several times throughout the night.

Non-Rapid Eye Movement (NREM) Sleep

1. Stage 1 (N1): This is the lightest stage of sleep, often considered the transition phase between wakefulness and sleep. It typically lasts for 5 to 10 minutes. During this stage, muscle activity slows down and slight twitching may occur.

2. Stage 2 (N2): This stage marks the onset of actual sleep. It lasts for about 20 minutes. Here, the heart rate slows, body temperature drops, and brain waves become slower with occasional bursts of rapid waves called sleep spindles.

3. Stage 3 (N3): Also known as deep sleep or slow-wave sleep, this stage is crucial for feeling refreshed in the morning. It generally lasts for 20-40 minutes. During N3, the body repairs tissues, builds bone and muscle, and strengthens the immune system.

Rapid Eye Movement (REM) Sleep

After NREM sleep, the cycle moves into REM sleep, which lasts about 10-20 minutes in the first cycle and lengthens with each cycle. This is where most dreaming occurs. REM sleep is characterized by increased brain activity, rapid eye movements, and temporary paralysis of the limb muscles. It plays a key role in learning and memory consolidation.

Hormones and Sleep

Several hormones play critical roles in regulating sleep:

1. Melatonin: Produced by the pineal gland, melatonin is often known as the "sleep hormone." It's regulated by light exposure; darkness triggers melatonin production, promoting sleepiness.

2. Cortisol: Often termed the "stress hormone," cortisol levels typically rise in the early morning to help you wake up and fall in the evening to facilitate sleep.

3. Growth Hormone: Released during deep sleep (stage N3), it plays a vital role in growth, body composition, cell repair, and metabolism.

4. Adenosine: Builds up in your blood while you are awake and decreases during sleep. It's thought to be one of the factors that promotes sleep pressure.

5. GABA (Gamma-Aminobutyric Acid): This neurotransmitter helps reduce neuronal excitability throughout the nervous system and is crucial for achieving deep sleep.

Factors Affecting Sleep Cycle

Several factors can affect the sleep cycle, including age, lifestyle habits, stress, diet, and exposure to light, particularly blue light from screens. Disruptions in these factors can lead to sleep disorders such as insomnia, sleep apnea, or circadian rhythm disorders.

Understanding the sleep cycle and the roles of various hormones can be crucial in maintaining good sleep hygiene and overall health. Practices like maintaining a regular sleep schedule, reducing screen time before bed, and creating a comfortable sleep environment can support healthy sleep cycles.