What Melatonin Actually Does
Melatonin is a neurohormone produced by the pineal gland, a small endocrine structure at the centre of the brain. Its primary role is not to induce sleep directly it is more accurate to describe it as a biological darkness signal. As ambient light decreases in the evening, melatonin secretion rises, communicating to the body that night has arrived and that physiological preparation for sleep should begin.
This rise called dim-light melatonin onset (DLMO) typically begins one to two hours before a person's habitual sleep time under normal light conditions. It triggers a cascade of downstream effects: core body temperature drops, alertness decreases, and sleep propensity increases. Melatonin does not force sleep; it shifts the body into the right state for sleep to occur naturally.
The Mechanism: How Blue Light Suppresses Melatonin
The ipRGC-SCN Pathway
The suppression of melatonin by light is not mediated by the rods and cones used for vision. It is driven by a separate class of retinal cells: intrinsically photosensitive retinal ganglion cells (ipRGCs), which contain the photopigment melanopsin. These cells are maximally sensitive to short-wavelength blue light in the 460–480 nm range precisely the range most heavily emitted by modern LED screens.
When ipRGCs detect blue light in the evening, they transmit signals via the retinohypothalamic tract (RHT) to the suprachiasmatic nucleus (SCN) the brain's master circadian clock. The SCN then suppresses melatonin production via a neural pathway to the pineal gland. The message is unambiguous: it is still daytime. Delay sleep.
The Dose-Response Relationship
This suppression is not binary it follows a dose-response curve. West et al. (2011) demonstrated that increasing irradiances of narrowband blue LED light (peak wavelength 469 nm) produced progressively greater plasma melatonin suppression in healthy subjects, with results fitting a sigmoidal fluence-response curve. Critically, narrow-bandwidth blue LED light was found to be more potent at suppressing melatonin than broad-spectrum white fluorescent light at comparable irradiances. West et al.
Brainard et al. (2001) had previously established the action spectrum for melatonin suppression in humans, identifying peak photosensitivity in the 446–477 nm range with a best-fit peak at approximately 464 nm. This work provided foundational evidence that a novel, non-rod, non-cone photopigment was responsible for this effect later confirmed to be melanopsin. Brainard et al.
What Happens to Sleep
Delayed Circadian Phase
When melatonin is suppressed by evening screen use, the body's circadian phase shifts later. Chang et al. (2015) demonstrated that participants using a light-emitting eReader before bed showed reduced evening sleepiness, suppressed melatonin secretion, later timing of their circadian clock, and reduced next-morning alertness compared to when reading a printed book. The effect was not limited to the night of exposure next-morning alertness was measurably impaired even after a full night of sleep, suggesting the circadian disruption extended beyond immediate melatonin suppression. Chang et al.
Reduced REM Sleep
The same study found that light-emitting device use before bed reduced not only the amount of REM sleep, but also delayed its timing within the sleep cycle. REM sleep is critical for memory consolidation, emotional regulation, and cognitive recovery. A consistent delay in its onset even by 30–60 minutes has measurable consequences for the quality of the following day. Chang et al.
The Alerting Effect
Blue light after dusk exerts a potent alerting effect, reinforcing wakefulness and circadian misalignment. Chronic exposure may cumulatively contribute to insufficient and irregular sleep, with downstream consequences for cognition, mood, and metabolic health. This alerting effect is part of the mechanism by which evening device use delays bedtime itself users feel more awake at a time when biology would otherwise be signalling wind-down.
Red Light as a Contrast Case
The difference between blue and red light at night illustrates the mechanism clearly. Teran et al. (2025) compared three-hour exposure to blue light (464 nm) versus red light (631 nm) in healthy adults from 9pm to midnight. Blue light maintained melatonin suppression with minimal recovery over the full three hours. Red light, by contrast, allowed a significant melatonin rebound after two and three hours, restoring secretion toward baseline levels. Teran et al.
This contrast is relevant because it suggests the wavelength not brightness alone is the primary driver of melatonin suppression at night.
What the Research Does and Does Not Say
The evidence for blue light's role in melatonin suppression and circadian phase delay is robust and well-replicated. What the research does not support is the claim that all evening light exposure is equally harmful, or that melatonin suppression inevitably leads to clinical sleep disorders. Individual variation in sensitivity is significant, and factors such as prior light history, age, and pupil size all modulate the response.
The clearest implication of the evidence is about timing and wavelength: short-wavelength light in the hours before bed, when melatonin would naturally be rising, is the specific combination that disrupts the body's wind-down signal most reliably.