Ever suddenly lose focus after working for more than an hour? Or feel your brain “drop off” even though it’s only late afternoon?
That’s not just ordinary fatigue. The human body is designed to operate in 90–120 minute energy cycles, followed by 20–30 minutes of recovery before it can focus again. Interestingly, this rhythm isn’t a modern concept, it mirrors the same biological pattern the body follows during sleep each night.
Sleep: A Quiet Process Full of Activity
Although it appears passive, sleep is one of the body’s busiest processes. It’s an unconscious state that remains internally active, the brain keeps working, the body repairs itself, and various internal systems continue maintaining balance.
Its exact purpose is not yet fully understood, but several major theories offer strong explanations.
- Inactivity Theory suggests that sleep is an evolutionary mechanism: animals that stayed still at night were safer from predators and environmental dangers.
- Energy Conservation Theory highlights that sleep reduces energy needs by around 10%, critical in times when hunting or foraging is inefficient.
- Restoration Theory proposes that the body repairs cells, grows tissues, produces proteins, and releases essential hormones during sleep.
- Lastly, Brain Plasticity Theory states that the brain undergoes restructuring during sleep, especially in infants who may sleep up to 14 hours a day because their brains are developing rapidly.
Taken together, these theories show that sleep is a fundamental biological activity.
The Sleep Cycle: A Repeating 90–120 Minute System
Every night, the body moves through a highly structured sleep pattern: Non–Rapid Eye Movement (NREM) followed by Rapid Eye Movement (REM). A single cycle lasts about 70–120 minutes, and a normal night includes four to five cycles.
During NREM sleep, the body passes through three stages:
- Stage 1 lasts only 1–7 minutes, very light sleep with alpha brain waves.
- Stage 2 goes deeper, marked by "sleep spindles" and "K-complexes", and can make up about 50% of total sleep. This stage is strongly linked to memory consolidation.
- Stage 3 is the deepest stage, characterized by slow, high-amplitude brain waves.
Only after these stages does the body enter REM sleep, the phase where dreams occur. Interestingly, the body shuts down almost all muscle activity so dreams do not translate into actual movements. The brain’s wave patterns here are unique: sawtooth, theta, and unsynchronized alpha waves.
And here’s the key: this 90–120 minute rhythm is the very same pattern as the body’s natural daytime energy cycle when we’re awake.
What Triggers Sleep? The Brain Has Two Regulatory Systems
Sleep doesn’t simply “happen.” It emerges from the interaction of two major systems:
- The homeostatic process, which increases sleep pressure the longer we stay awake.
- The circadian rhythm, a roughly 24.2-hour biological clock that adjusts to light through the retina.
Internally, sleep begins when the ventrolateral preoptic nucleus (VLPO) suppresses arousal centres such as the locus coeruleus and dorsal raphe. The REM phase is triggered by REM-on neurons in the pontine region.
Melatonin is released as light diminishes, helping lower the body’s activity rhythm. Body temperature also drops in the morning and rises at night, following this circadian cycle.
These two systems operate like an orchestra, determining when the body is ready to sleep and when it should remain awake.
When Sleep Rhythms Are Disrupted
Several conditions can interfere with healthy sleep:
- Insomnia, often triggered by stress or excessive stimulation.
- Obstructive Sleep Apnea (OSA), where the airway becomes blocked and breathing repeatedly stops; its primary treatments include CPAP or BiPAP.
- Central Sleep Apnea, caused by disruptions in the brain’s respiratory control centres.
- Mixed Sleep Apnea, combining both forms.
- Hormonal imbalance of leptin and ghrelin triggered by sleep deprivation, increasing appetite and contributing to weight gain.
- Narcolepsy, where orexin-producing neurons are lost, leading to extreme sleepiness and cataplexy.
- Somnambulism, or sleepwalking, which occurs when sleep and wake states overlap.
Each of these disorders illustrates how sensitive the body is to even small shifts in sleep rhythms.
The 90–120 Minute Rhythm Is Everywhere
The sleep cycle shows that the body functions in short blocks of activity before requiring recovery. This is the biological reason human focus is typically optimal for 90–120 minutes at a time. After that, the body signals the need for a break.
This rhythm is a reminder that the body is not designed for nonstop hours of work. The body works in cycles. Sleep works in cycles. Energy works in cycles.
Source: Brinkman, J. E., Sharma, S., & Reddy, V. (2023). Physiology of Sleep. PubMed; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK482512/
