When the lights go down, many of us reach for our phones. The question of whether that habit is truly harmful has moved from alarmist headlines to more nuanced science: the effect of screens on sleep depends on three things—how bright the screen is, when you use it, and what you are doing on it.

Laboratory studies decades ago established the biology. Special retinal cells called intrinsically photosensitive retinal ganglion cells, or ipRGCs, contain the photopigment melanopsin and are most sensitive to blue light near 480 nanometers. Activation of this pathway suppresses melatonin, the hormone that signals the brain to prepare for sleep, and shifts circadian timing. “We know the mechanism,” said Dr. Ana Morales, a chronobiologist at the University of California, San Diego. “Light at night tells your internal clock to stay awake.”

But controlled-lab exposure does not map directly onto nightly habits. Recent clinical trials and field studies indicate that brief, dimmed use of a phone—particularly more than an hour before habitual bedtime—produces little measurable impact on sleep. Conversely, prolonged exposure to bright, blue-rich light within the two hours before sleep, or engaging with emotionally arousing content, reliably delays sleep onset and reduces sleep quality in many individuals.

Researchers distinguish laboratory melatonin suppression studies from real-world trials that use wrist actigraphy and polysomnography to track sleep. “In the lab, we can show that three hours of bright blue light delays melatonin and sleep timing,” said Dr. James Huang, a sleep medicine specialist. “In everyday life, the dose and the content matter. Ten minutes of reading a quiet article at low brightness is not the same as an hour of doomscrolling in a dark room.”

Technology makers have responded: Apple’s Night Shift, Android’s Blue Light filter, adaptive warm-toning and automatic brightness settings attempt to reduce melanopic stimulation. Evidence suggests these measures lower the biological potency of screen light but do not eliminate the behavioral risks of late-night use. Several randomized trials have found modest improvements in sleep onset or subjective sleep quality with night modes, but the largest effects came from behavioral changes—setting device curfews, enabling wind-down modes that limit app notifications, and substituting low-light activities such as reading printed books.

Population groups differ. Adolescents and young adults, whose circadian systems are already shifted later, show larger sleep deficits with late-night screen use. Older adults tend to be less sensitive to light’s phase-shifting effects but may still experience sleep fragmentation if devices disrupt sleep routines. Experts warn that people working irregular hours or with mood disorders should be especially cautious.

The policy and public-health challenge is to translate these nuanced findings into practical guidance. “Sensible rules—dim your screen, shift to warmer tones, avoid stimulating content in the last hour—are low-cost and effective for most,” Morales said. “At the same time, clinicians must recognize who needs stronger interventions.”

For most people, looking at a phone briefly before bed is likely fine when done with low brightness and calm content. But technology alone is not the cure; behavior remains the decisive factor in protecting sleep.