Sleep Stages Explained

The sleep stages are broadly divided into two main types: Non-Rapid Eye Movement (NREM) sleep and REM (Rapid Eye Movement). NREM sleep usually accounts for about 70-75% of total sleep time while REM sleep accounts for the rest of 20-25% of total sleep time.
There are a total of five stages of sleep. Of which, one is wakefulness/alert, three stages belong to NREM sleep and one is for REM sleep. Each of these stages is defined by unique patterns of brain activity and physiological changes. During sleep, these stages do not occur in any single particular sequence. As mentioned earlier, each sleep cycle is of 90-110 minutes and repeats throughout the night, forming the building blocks of a restorative sleep experience.

The following are the five stages of sleep:

Stage W (Wakefulness)
Stage 1 (N1)
Stage 2 (N2)
Stage 3 (N3)
Stage 4 (REM Sleep)

What Is Sleep?

Sleep is a naturally recurring process characterized by altered consciousness, relatively inhibited sensory activity, reduced muscle activity, and a decreased ability to interact with the surroundings.
During sleep, the body performs critical tasks essential for our well-being. According to the National Heart, Lung, and Blood Institute (NHLBI), sleep supports healthy brain function and physical health. Experts believe that sleep is a time for tissue repair, memory consolidation, and the resetting of key physiological systems.

Why Do We Sleep?

While the complete picture is still being assembled by sleep research, several key theories aim to explain why humans need to sleep. Few leading theories include:
Energy Conservation: By reducing metabolic rate and body temperature, sleep helps conserve energy that was expended during waking hours. This was particularly crucial for our ancestors when food was scarce.
Restoration and Repair: Sleep is prime time for the body to repair itself. During deep sleep, the body releases growth hormone, which facilitates the repair of muscles and tissues. It also helps bolster the immune system, making us better equipped to fight off infections.
Brain Plasticity: The brain plasticity theory posits that sleep is critical for brain development and function. It allows neurons to reorganize, playing a vital role in learning and memory consolidation. During sleep, the brain clears out waste products, like beta-amyloid, that accumulate during waking hours. This hypothetical function of sleep has been linked to preventing cognitive decline.

How Much Sleep Do We Need?

It’s easy to treat sleep as a numbers game, however, the total number of hours slept is only part of what leaves you feeling restored. The quality of your sleep is also important.
According to the National Sleep Foundation, the recommended sleep duration varies across age groups. For instance, for newborns (0–3 months) it is around 14 to 17 hours, for infants (4–11 months) it is 12 to 15 hours, and for toddlers (1–2 years) it is 11 to 14 hours. Preschoolers (3–5 years) are recommended 10 to 13 hours of sleep, school-aged children (6–13 years) 9 to 11 hours, and teenagers (14–17 years) 8 to 10 hours. Young adults (18–25 years) and adults (26–64 years) are recommended 7 to 9 hours of sleep per night, while older adults (65+ years) 7 to 8 hours.
Consistent inability to meet these recommended sleep targets could lead to sleep deprivation. This increases the risk of impaired cognitive function, weak immune system, and other chronic diseases like high blood pressure, heart disease, and diabetes.

Introduction to Sleep Cycle

Sleep is not a monolithic state. Instead, it progresses through a repeating pattern known as the sleep cycle. Each cycle is composed of several distinct stages of sleep and typically lasts about 90 to 110 minutes. Over the course of a full night, most adults may experience up to three to five of such sleep cycles.
This entire process is governed by the internal body clock called the Circadian Rhythm. Each of these circadian rhythm cycles usually lasts up to 24.2 hours in humans. It regulates different body functions including sleep. Circadian rhythm is controlled by the master orchestrator of sleep in the brain called the suprachiasmatic nucleus — an area found in the part of the brain called hypothalamus.

Organ Systems Behind Sleep

Several brain structures work together to manage the transition between sleep and wakefulness: Hypothalamus: Signals the body when it is time to fall asleep.
Hippocampus: It is the memory center which is especially active during dreaming.
Amygdala: Also known as emotion center. It processes emotions during sleep and is one of the reasons why dreams can feel so intense.
Thalamus: It inhibits sensory signals from travelling to brain’s cortex allowing people to stay asleep
Reticular formation: Manages the shift from sleep to wakefulness.
Pons: Assists in REM sleep onset. PPRF (paramedian pontine reticular formation/conjugate gaze center) activity causes the eye movements seen in REM sleep.
The structure of each sleep cycle, including the amount of time spent in each stage, may also vary from night to night. This is known as our "sleep architecture". Stay tuned to learn about sleep architecture in our next blog!

Disclaimer

This is an educational blog and not medical advice. If you or your loved ones suffer from sleep issues, call Telemedora at 650-687-7368 or visit telemedora.com to find out how we can help you. Whether it's sleep apnea, insomnia, or just restless nights, Telemedora brings expert care directly to your home. We help patients get the rest they need so they can wake up refreshed, focused, and ready to crush their goals for the next day.