Sleep phases: what happens in each stage of rest

Sleep is not a uniform state. During the hours you spend in bed, your brain goes through a well-defined sequence of 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 them is 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 difficult to concentrate, or why your mood fluctuates for no apparent reason. In our sleep and rest section, you will find more information on the environmental factors that influence the quality of rest. 

Sleep architecture: cycles, phases, and duration

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

Sleep phases explained one by one 

NREM Phase 1 — The transition to sleep 

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

NREM Phase 2 — Light sleep 

Phase 2 accounts for approximately 50% of total sleep time in an adult. 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. 

NREM Phase 3 — Deep or slow-wave sleep 

This is the most restorative phase 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 phase is shortened or fragmented, the body does not have enough time to complete its repair cycles. The result is accumulated fatigue that does not go away with rest, greater susceptibility to infections, and poorer regulation of weight and mood. The effects of incomplete rest on overall health go far beyond superficial tiredness. 

REM Phase — The sleep of dreams 

REM (Rapid Eye Movement) sleep is when vivid dreams occur. The brain reaches a level of activity comparable to the waking state, while the body experiences temporary muscle paralysis. It is in this phase that emotional and procedural memory are consolidated, the day's 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 alarm clocks have a disproportionate impact on emotional stability and cognitive function the next day. 

Environmental factors that can interrupt sleep phases 

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 of mobile phones, tablets, computers, and most LED bulbs emit precisely this type of light—so-called blue light—even in the hours before going to bed. 

When the brain receives this light signal, it interprets it as still being daytime and inhibits the production of melatonin. Research published in journals such as the Journal of Clinical Endocrinology & Metabolism has documented this mechanism consistently. The result is a delay in sleep onset and a compression of deep and REM sleep phases in the first 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 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 the IGEF (Institut für Geopathologie) and clinical trials conducted with SPIRO® technology from Noxtak—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 scientific studies available in our research section. 

Dirty electricity in the home 

Less known is so-called dirty electricity: high-frequency interference circulating through the domestic electrical grid, generated by light dimmers, switched-mode power supplies, and solar installation inverters. This type of interference can propagate through wiring even when all wireless devices are 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 phase: practical guide 

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

• Adopt a light routine consistent with your biology. Replace artificial nighttime light sources with lighting without a blue component at least 90 minutes before going to bed. This allows melatonin to start being released on time and for Phase 1 of sleep to be reached without resistance. Amber bulbs and the Amber lamp are specifically designed for this purpose, eliminating the range of wavelengths that inhibit melatonin. You can learn more about the studies that support this type of lighting in our section on studies on healthy lighting.
• Reduce emissions in the bedroom. Turn off the WiFi router at night and put your mobile phone in airplane mode or in another room. These are zero-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—the results of which you can consult in our studies section. For bedroom use, the SPIRO® Disc is sized to cover the environment of a complete room; the SPIRO® Square is suitable for medium-power devices like the router.
• Consider the dirty electricity of your installation. The Stroom Master Pro acts on the domestic electrical grid by 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 individual effects on sleep depend on the exposure profile of each home.
• Maintain stable temperatures and schedules. The deep sleep of Phase 3 is promoted by temperatures between 16 and 19°C. Sudden changes in temperature or irregular schedules disrupt cycle architecture and reduce the proportion of REM sleep. 

Frequently asked questions about sleep phases 

How many phases does sleep have and how long does each last? 

Sleep has four phases: 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 phase varies throughout the night: deep sleep predominates in the first half and REM in the second. 

What happens if all sleep phases are not completed? 

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

Does having the WiFi on at night affect sleep phases? 

Some studies have explored possible associations between exposure to RF EMFs and changes in slow-wave sleep, although the evidence is not yet conclusive. Turning off the router at night is a reasonable and zero-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 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 the onset of sleep. This not only makes it take longer to fall asleep, but it compresses the deep and REM sleep phases 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 backed interventions available. Discover more about what blue light is and how it affects your health, or explore our range of healthy lighting for the bedroom.