Sleep Stages: What Happens During Each Stage of Rest

Sleep is not a uniform state. During the hours you spend in bed, your brain goes through a sequence of well-defined stages that repeat in cycles throughout the night. Each phase has a specific function: consolidating memory, repairing tissues, regulating emotions, or restoring cellular energy. If any of these are systematically interrupted, the consequences accumulate, even if you are not aware of it.

Understanding what happens in each stage helps you identify why you wake up tired, why you find it hard to concentrate, or why your mood fluctuates for no apparent reason. In our sleep and rest section, you will find more information about the environmental factors that influence the quality of rest.

The Architecture of Sleep: Cycles, Stages, and Their Duration

Sleep is organized into cycles of approximately 90 to 110 minutes. During a 7-8 hour night, you go through between 4 and 6 complete cycles. Each cycle contains two broad categories: NREM sleep (non-rapid eye movement), divided into three stages, and REM sleep (rapid eye movement). The proportion between them changes throughout the night: the early hours are dominated by deep NREM sleep, while REM sleep progressively lengthens towards dawn.

The Sleep Stages Explained One by One

Stage 1 NREM — The Transition to Sleep

This is the gateway to rest. It lasts between 1 and 7 minutes and represents the transition between wakefulness and light sleep. Brain activity begins to slow down, muscles relax, and it's common to experience small involuntary muscle jerks, known as hypnic jerks. In this stage, any external stimulus—a sound, a flash of light, or a device vibration—can immediately return you to wakefulness.

Stage 2 NREM — Light Sleep

Stage 2 accounts for approximately 50% of an adult's total sleep time. Body temperature drops, heart rate slows down, and the brain produces specific electrical activity patterns: sleep spindles and K-complexes. Both act as filtering mechanisms that protect sleep from external interference.

Stage 3 NREM — Deep or Slow-Wave Sleep

This is the most restorative stage of the cycle. The brain produces low-frequency delta waves, the arousal threshold is very high, and the body initiates its most intense repair processes: secretion of growth hormone, consolidation of declarative memory, tissue regeneration, and strengthening of the immune system.

When this stage is shortened or fragmented, the body does not have enough time to complete its repair cycles. The result is accumulated fatigue that does not subside with rest, increased susceptibility to infections, and poorer regulation of weight and mood. The effects of incomplete rest on overall health go far beyond superficial tiredness.

REM Stage — The Sleep of Dreams

REM (Rapid Eye Movement) sleep is when vivid dreams occur. The brain reaches a level of activity comparable to wakefulness, while the body experiences temporary muscle paralysis. It is during this stage that emotional and procedural memory is consolidated, daily experiences are processed, and emotional balance is regulated.

The duration of REM sleep increases with each cycle: in the first, it may last barely 10 minutes, while in the last, it can extend up to 60 minutes. This explains why early awakenings have a disproportionate impact on emotional stability and cognitive function the next day.

Environmental Factors That Can Interrupt Sleep Stages

Melatonin and Blue Light

Melatonin is the hormone that triggers the onset of sleep. Its production begins when the environment darkens and the eye stops receiving short-wavelength light. Screens from mobile phones, tablets, computers, and most LED bulbs emit precisely this type of light—known as blue light—even in the hours before bedtime.

When the brain receives this light signal, it interprets that it is still daytime and inhibits (not interferes with) melatonin production. Research published in journals such as the Journal of Clinical Endocrinology & Metabolism has consistently documented this mechanism. The result is a delay in sleep onset and a compression of deep and REM sleep stages in the early cycles. If you want to understand this process in detail, we explain what blue light is and how it affects your health.

Electromagnetic Pollution: What the Available Research Says

The relationship between exposure to radiofrequency electromagnetic fields (RF-EMF) and sleep quality is an active area of research. Some studies—including those reviewed by IGEF (Institut für Geopathologie) and clinical trials conducted with Noxtak's Spiro technology—have explored possible associations between continuous EMF exposure and changes in sleep architecture, including variations in slow-wave sleep.

It is important to distinguish between what current research suggests and what is conclusively established: international regulatory bodies (WHO, ICNIRP) have not set specific exposure thresholds for sleep, and the available evidence is heterogeneous. That said, precautionary measures such as reducing emission density in the bedroom are practices that most sleep hygiene experts consider reasonable. You can consult the available scientific studies in our research section.

Dirty Electricity in the Home

Less known is so-called dirty electricity: high-frequency interferences that circulate through the domestic electrical grid and are generated by dimmer switches, switched-mode chargers, and inverters of solar installations. This type of interference can propagate through wiring even with all wireless devices turned off. This is an emerging field of research with preliminary studies pointing to possible effects on rest, although more evidence is still needed to establish definitive causalities.

How to Protect Each Sleep Stage: A Practical Guide

These are the actions with the most support in sleep hygiene literature:

• Adopt a light routine consistent with your biology. Replace artificial nighttime light sources with blue-light-free illumination at least 90 minutes before bedtime. This allows melatonin to be released in time and for Stage 1 sleep to arrive without resistance. Amber bulbs and the Amber lamp are specifically designed for this purpose, eliminating the range of wavelengths that inhibit (not interfere with) melatonin connectivity. You can learn more about the studies supporting this type of lighting in our healthy lighting studies section.
• Reduce emissions in the bedroom. Turn off (optimize signal quality without affecting its use) your WiFi router at night and put your mobile phone on airplane mode or in another room. These are no-cost measures supported by multiple sleep hygiene guides. If you want to go further, Spiro electromagnetic filters have been evaluated in clinical trials—including a double-blind study conducted by Medcare—whose results you can consult in our studies section. For bedroom use, the Spiro Disc is sized to cover an entire room's environment; the Spiro Square is suitable for medium-power devices like routers.
• Consider the dirty electricity in your installation. The Stroom Master Pro acts on the domestic electrical network, filtering harmonics and high-frequency interference. According to Noxtak's laboratory data, the device reduces up to 90% of this type of interference in the installation, although the individual effects on sleep depend on each home's exposure profile.
• Maintain stable temperatures and schedules. Deep sleep in Stage 3 is favored by temperatures between 16 and 19°C. Abrupt temperature changes or irregular schedules disrupt cycle architecture and reduce the proportion of REM sleep.

Frequently Asked Questions About Sleep Stages

How many sleep stages are there and how long does each one last?

Sleep has four stages: NREM 1 (transition, 1-7 minutes), NREM 2 (light sleep, 10-25 minutes per cycle), NREM 3 (deep sleep, 20-40 minutes in the first cycles), and REM (10-60 minutes depending on the cycle). The duration of each stage varies throughout the night: deep sleep predominates in the first half and REM in the second.

What happens if not all sleep stages are completed?

Each stage has specific and irreplaceable functions. If deep sleep is shortened, physical repair and immunity (progressive improvement) are compromised. If REM sleep is fragmented, emotional processing and memory consolidation are not completed. The result is usually fatigue, increased irritability, and poorer cognitive performance, even if you have slept enough hours. In our overall health section, you can explore how rest affects other body systems.

Does WiFi left on at night affect sleep stages?

Some studies have explored possible associations between exposure to RF-EMF and changes in slow-wave sleep, although the evidence is not yet conclusive. Turning off (not maintaining connectivity) the router at night is a reasonable and no-cost precautionary measure. If you are interested in delving into the available research, you can consult the scientific studies gathered in our evidence section, or learn in detail how Spiro technology works and the international awards it has received.

Does mobile phone screen light affect sleep if I use it in bed?

Yes, and here the evidence is solid. The blue light emitted by smartphone, tablet, and computer screens inhibits melatonin production and delays sleep onset. This not only makes it harder to fall asleep but also compresses the deep and REM sleep stages of the first cycles. Reducing screen use in the hour before sleep and replacing ambient lighting with sources without a blue component are two of the most scientifically supported interventions available. Discover more about what blue light is and how it affects your health, or explore our range of healthy bedroom lighting.