What your sleep tracker reveals about the cause

How sleep is structured

A normal night consists of roughly 4–6 sleep cycles, each 90–110 minutes long. Every cycle passes through three phases: light sleep, deep sleep (slow-wave sleep) and REM sleep.

These phases are not randomly distributed across the night – they follow a characteristic pattern:

• First half of the night: dominated by deep sleep. Sleep pressure (Process S) is highest here, which produces especially deep, regenerative sleep stages.
• Second half of the night: increasingly dominated by REM sleep. As sleep pressure dissipates, the circadian clock begins to promote wakefulness, and the brain switches more time into REM.

This pattern is important for interpreting tracker data: depending on which phase – and in which half of the night – is disrupted, the cause differs significantly.

Heat: deep sleep suffers most

Excess temperature in the sleeping environment primarily attacks deep sleep in the first half of the night. This is precisely the phase in which the body most needs to dissipate heat to reach deep slow-wave sleep.

When the microclimate under the duvet is too warm, thermoregulatory neurons in the hypothalamus suppress slow-wave activity. Sleep becomes shallower, transitions from light to deep sleep fail or are shortened. REM sleep is usually less severely affected – at least initially – since it does not depend as directly on core cooling.

Typical tracker pattern for heat: Deep sleep markedly reduced, especially in the first 2–3 hours. Multiple short wake-ups. REM sleep roughly normal or only slightly reduced.

Stress and elevated cortisol

Psychological stress – acute (argument, exam, bad news) or chronic (work pressure, worry) – activates the hypothalamic–pituitary–adrenal axis and leads to elevated cortisol levels. Cortisol is a waking hormone: it suppresses the deep sleep-promoting adenosine signal and directly inhibits the slow-wave generator in the thalamus.

Unlike heat, stress affects the entire night: sleep onset is delayed, transitions into deep sleep are shortened throughout the night, and early wake-ups become more frequent. REM sleep is also affected – REM is sensitive to the balance of noradrenaline and serotonin, which stress alters.

Typical tracker pattern for stress: Prolonged sleep onset, reduced deep sleep throughout the night, shortened or fragmented REM sleep, frequent awakenings. Often the overall sleep duration is also reduced.

Alcohol

Alcohol has a distinctive and often misunderstood impact on sleep. It is sedating but not sleep-promoting in the physiological sense – and the tracker exposes this difference clearly.

In the first half of the night, alcohol suppresses REM sleep almost completely (Roehrs & Roth 2001). The body sleeps heavily, but this is not the same as healthy deep sleep: normal slow-wave activity is present but REM is nearly absent. In the second half of the night, as alcohol is metabolised, REM sleep rebounds strongly – often producing vivid dreams and early waking.

Typical tracker pattern for alcohol: REM strongly reduced in the first half of the night, deep sleep initially elevated or normal, then pronounced REM rebound in the second half of the night with frequent awakenings and early waking.

Going to bed too late

The timing of sleep relative to the circadian rhythm matters greatly. Going to bed late – after the circadian window for deep sleep has already narrowed – truncates the deep sleep phases that would normally occur early in the night.

Because the circadian clock begins to promote wakefulness from the early morning hours regardless of sleep timing, a late bedtime compresses the available sleep window from both ends: the first deep-sleep opportunity is missed or shortened, and waking occurs at the same clock time as usual.

Typical tracker pattern for going to bed too late: First sleep cycle reduced or missing, overall deep sleep reduced, REM sleep roughly proportional but the total sleep period is shortened.

Screens before bed

Blue-light exposure from screens in the hours before sleep suppresses melatonin secretion from the pineal gland (Chang et al. 2015). Melatonin is not a direct sleep initiator but a darkness signal that shifts the circadian phase and lowers the arousal threshold. Without adequate melatonin suppression of wakefulness, sleep onset is delayed.

The subsequent sleep is structurally normal in composition but is delayed in onset and often shortened if an alarm terminates it at the usual time.

Typical tracker pattern for screen use: Delayed sleep onset, otherwise relatively normal sleep architecture proportionally – but shortened total sleep duration and consequent reduction in late-night REM.

Reading the pattern – summary

How the pattern helps find the cause

The tracker alone cannot diagnose anything – it measures movement, heart rate and sometimes skin temperature indirectly and infers sleep stages from that. These inferences have margins of error, and single-night variation is normal. What the pattern delivers is a tendency over multiple nights.

If you notice over several nights that your deep sleep is consistently low but your REM sleep is largely intact, thermal factors – too warm sleeping environment – become a plausible candidate. If both deep and REM sleep are consistently reduced with a prolonged sleep onset, stress is a more likely cause. If REM is strongly suppressed on evenings when you drank but rebounds towards morning, the alcohol effect is clearly visible.

The goal is not precise single-night diagnosis but recognising patterns that point you towards a testable hypothesis. Adjusting one variable (e.g. a lighter duvet on evenings after exercise) and observing whether the pattern changes is a meaningful next step.

Scientific sources for this article

Tsuzuki et al. (2004) – Sleep Stage Distribution Depending on Ambient Temperature

Excess temperature in the sleep environment primarily disrupts deep sleep phases in the first half of the night. Deep sleep fell by up to 43 %, REM by 21 %.

Thermal stressDeep sleep

Roehrs & Roth (2001) – Sleep, Sleepiness, and Alcohol Use

Alcohol suppresses REM sleep in the first half of the night and causes a rebound in the second half. The sedative effect does not equate to physiologically normal sleep.

AlcoholREM rebound

Buckley & Schatzberg (2005) – On the Interactions of the Hypothalamic–Pituitary–Adrenal Axis and Sleep

Review: cortisol and the HPA axis suppress deep sleep and fragment sleep architecture in psychological stress and anxiety disorders.

StressCortisol

Czeisler et al. (1980) – Human Sleep: Its Duration and Organization Depend on Its Circadian Phase

Seminal work showing that sleep timing relative to the circadian cycle determines deep sleep and REM distribution. Going to bed too late truncates early deep-sleep cycles.

Circadian timingSleep architecture

Chang et al. (2015) – Evening Use of Light-Emitting eReaders Negatively Affects Sleep

Randomised crossover study (PNAS). Blue light from screens before bed suppresses melatonin, delays sleep onset and reduces next-morning alertness.

Blue lightMelatonin