Light, screens and sleep.

Light sets the clock

Your body clock runs on light. Light in the evening and at night pushes it later; light in the morning pulls it earlier (Khalsa, 2003). After a week of camping with only natural light, people's evening melatonin, the hormone that rises before sleep, started about two hours earlier (Wright, 2013).

Evening screens and blue light

Ordinary room light in the hours before bed delays melatonin in almost everyone (Gooley, 2011), and some people are far more sensitive to it than others (Phillips, 2019). Screens add to that. In a lab study, reading on a tablet for about four hours a night for five nights made people take about ten minutes longer to fall asleep than reading a printed book (Chang, 2015).

In everyday use the effect is smaller. Half an hour on a tablet (Grønli, 2016) or an hour on a bright screen (Heath, 2014) did not change how long people took to fall asleep. In a survey of 122,000 US adults, daily screen use in the hour before bed went with about eight minutes less sleep on work nights and later bedtimes (Zhong, 2025). What you do on the screen, and how late it keeps you up, may matter as much as the light.

Morning and daytime light

Daylight early in the day helps keep the clock on time. An expert panel recommends bright days, dim evenings and dark nights (Brown, 2022). In a UK study of 88,905 adults who wore light sensors, brighter days and darker nights went with a lower risk of dying during the study (Windred, 2024): an association, not proof, but it points the same way.

Blue-blocking glasses

A Cochrane review found the effect of blue-blocking glasses on sleep uncertain, with very low certainty (Singh, 2023). Another review found small, mixed benefits, mostly in people with insomnia or other sleep and mood conditions (Shechter, 2020). Night modes on phones have not shown much either: in a trial of 167 young adults, sleep was the same with Night Shift on, off, or with no phone at all (Duraccio, 2021).

What to try

Get outdoor light in the morning, even on a cloudy day. Dim the lights in the last hour or two before bed. Put the phone down at a set time, more for the bedtime than for the colour. Keep the bedroom dark. None of these is a cure; together they make the clock's job easier, and your own sleep log will show whether they help you.