When we sleep well, we wake up feeling refreshed and alert for our daily activities. Sleep affects how we look, feel and perform on a daily basis, and can have a major impact on our overall quality of life.

To get the most out of our sleep, both quantity and quality are important. Teens need at least 8 hours—and on average 9¼ hours—a night of uninterrupted sleep to leave their bodies and minds rejuvenated for the next day. If sleep is cut short, the body doesn’t have time to complete all of the phases needed for muscle repair, memory consolidation and release of hormones regulating growth and appetite. Then we wake up less prepared to concentrate, make decisions, or engage fully in school and social activities.

How Does Sleep Contribute to All of These Things?

Sleep architecture follows a pattern of alternating REM (rapid eye movement) and NREM (non-rapid eye movement) sleep throughout a typical night in a cycle that repeats itself about every 90 minutes.

What role does each state and stage of sleep play?

NREM (75% of night): As we begin to fall asleep, we enter NREM sleep, which is composed of stages 1-4

Stage 1

• Between being awake and falling asleep
• Light sleep

Stage 2

• Onset of sleep
• Becoming disengaged from surroundings
• Breathing and heart rate are regular
• Body temperature drops (so sleeping in a cool room is helpful)

Stages 3 and 4

• Deepest and most restorative sleep
• Blood pressure drops
• Breathing becomes slower
• Muscles are relaxed
• Blood supply to muscles increases
• Tissue growth and repair occurs
• Energy is restored
• Hormones are released, such as: Growth hormone, essential for growth and development, including muscle development

REM (25% of night): First occurs about 90 minutes after falling asleep and recurs about every 90 minutes, getting longer later in the night

• Provides energy to brain and body
• Supports daytime performance
• Brain is active and dreams occur
• Eyes dart back and forth
• Body becomes immobile and relaxed, as muscles are turned off

In addition, levels of the hormone cortisol dip at bed time and increase over the night to promote alertness in morning.

Sleep helps us thrive by contributing to a healthy immune system, and can also balance our appetites by helping to regulate levels of the hormones ghrelin and leptin, which play a role in our feelings of hunger and fullness. So when we’re sleep deprived, we may feel the need to eat more, which can lead to weight gain.

The one-third of our lives that we spend sleeping, far from being “unproductive,” plays a direct role in how full, energetic and successful the other two-thirds of our lives can be.

Source: https://sleepfoundation.org/how-sleep-works/what-happens-when-you-sleep