How Day Length Sets the Body's Seasonal Clock

The clock that reads the sky

Every cell in the human body carries a roughly 24-hour molecular clock, but that clock drifts — left alone in constant light it typically runs a little longer or shorter than 24 hours. What keeps it locked to the actual day is photoperiod: the daily pattern of light and dark, sensed not by the rods and cones used for vision but by a separate population of retinal ganglion cells containing the photopigment melanopsin, most sensitive to blue light around 480 nanometres. Their signal travels a direct path, the retinohypothalamic tract, straight to the suprachiasmatic nucleus (SCN) in the hypothalamus — the body’s master clock.

The SCN does not control melatonin directly by simple on/off logic; it relays timing information through a multi-synaptic pathway down to the pineal gland, which secretes melatonin only during darkness and is suppressed the moment sufficient light reaches the retina. The result is a melatonin curve that rises a couple of hours before habitual sleep onset, peaks in the middle of the biological night, and falls sharply near natural wake time — effectively an internal signature of how long the night is.

Why day length itself is the seasonal signal

The duration of the melatonin secretion window scales with night length, and night length is a direct geometric consequence of Earth’s 23.4-degree axial tilt combined with latitude. Near the equator day length barely changes across the year; at 50 to 60 degrees latitude, winter days can shrink to eight hours or less while summer days stretch past sixteen. Many seasonally breeding animals (sheep, hamsters, some birds) read this melatonin duration as their photoperiod calendar, timing reproduction, moult and migration off it. Humans retain the same underlying circuitry, even though our behaviour rarely depends on it as strongly.

melatonin_duration ∝ night_length(latitude, day_of_year)
night_length = f( axial_tilt = 23.4°, solar_declination(day_of_year), latitude )
phase_delay_winter: melatonin onset and offset both shift later relative to clock time

Seasonal affective disorder as a phase problem

Seasonal affective disorder (SAD) is strongly associated with short winter photoperiods, and one of the leading explanatory models — the phase-shift hypothesis developed largely from Alfred Lewy’s work — proposes that in vulnerable individuals, shortened winter daylight delays the internal clock relative to the external clock and to the person’s normal wake time. The person then wakes while their internal night, and elevated melatonin, is still ongoing — a mismatch that correlates with the low mood, oversleeping, carbohydrate craving and fatigue typical of winter-pattern SAD. Northern latitudes with the shortest, most compressed winter photoperiods show the highest prevalence, consistent with a dose-response relationship to day length rather than temperature or weather alone.

Light therapy: correcting the phase, not just brightening the room

Bright light therapy (commonly 10,000 lux for 20 to 30 minutes) works best delivered in the morning, shortly after habitual wake time, because light exposure early in the biological night advances the clock — pulling the delayed rhythm back into alignment with the external day and shortening the internal melatonin window to match. The same light delivered in the evening can instead delay the clock further, which is why timing, not just intensity, determines whether light therapy helps or backfires. This is formalised by the phase response curve: light before the internal temperature minimum (roughly the second half of the biological night) delays the clock, light after it advances the clock.

What the simulation lets you explore

Sliding latitude and calendar date recomputes actual day length from solar geometry, then propagates it through a simplified melatonin-onset model so you can watch the secretion window widen in winter and narrow in summer, and see how a simulated light-therapy pulse shifts the phase back toward alignment — the same lever clinicians pull when treating winter-pattern depression.

Frequently asked questions

Try it live

Everything above runs in your browser — open Seasonal Rhythms and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.