Sunlight and the body clock
Your body knows what part of the day it is without your looking at a clock. The strongest dial on that inner clock is the daylight reaching your eyes, and morning light most of all.
Short answer
Our bodies have an internal clock of roughly 24 hours that governs sleep, wakefulness, hormones and body temperature; this is called the circadian rhythm. Left to itself the clock drifts slightly, and the strongest signal that resets it each day to the Earth's day is the daylight the eye detects.
Circadian rhythm: the clock inside us
In the brain, just above the point where the optic nerves cross, a small cluster of cells acts as the master clock. It ties sleep and wakefulness, body temperature, the timing of certain hormones and processes such as digestion to a cycle of about 24 hours. The research that illuminated the genetic and cellular workings of this mechanism was the subject of the 2017 Nobel Prize in Physiology or Medicine. An interesting feature of the internal clock is that it is not exactly 24 hours; in most people it is close to that but somewhat different. This is why it has to be resynchronised with the outside world every day.
Light: the clock's strongest setter
The signals that synchronise the internal clock with the day are called time-givers; meal times and movement play a part, but light is by a wide margin the strongest. In the retina there are specialised cells devoted not to vision but to measuring light levels. These cells are particularly sensitive to the blue-weighted component of daylight and pass their signals straight to the master clock. Light information also regulates the secretion of melatonin, sometimes called the hormone of darkness: bright light suppresses melatonin, while darkness allows secretion to begin.
The special role of morning light
Light's effect on the clock depends on the time of day it is received. Morning light pulls the clock earlier: you become sleepy sooner in the evening and waking the next morning is easier. Strong light late at night pushes the clock later. There is also a difference of scale. Even on an overcast day, outdoors is many times brighter than a typical indoor space. A well-lit office is at the level of a few hundred lux, while outdoors the figure runs into thousands, and on a sunny day into tens of thousands. Your eye smooths the difference so you do not feel it, but your internal clock knows it very well.
Evening light and screens
Long exposure to bright light in the evening, and especially to screen light at close range, can delay melatonin secretion and make falling asleep harder. The size of the effect varies from person to person and with screen brightness; it would not be correct to say that everyone who uses a screen in the evening will sleep badly. A balanced approach is to be generous with daylight and measured with light at night: softening household lighting in the evening and moving away from bright screens in the hour or so before bed are reasonable starting points.
Latitude, season and the light budget
How much daylight you can get depends on the latitude you live at and on the season. Near the equator daylight stays around 12 hours all year, while at the middle latitudes where Türkiye lies it rises to about 15 hours in summer and falls to 9 or 10 hours in winter; the gap widens further towards the poles. In the winter months the day is not only shorter, the light is also weaker because the Sun stays low on the horizon; this narrowing light budget is thought to play a part in why many people feel more sluggish and less motivated in winter. For those who experience marked low mood with a seasonal pattern, light-based support may be considered with a specialist. Modern life spent indoors narrows the same budget regardless of season, which is why the habit of getting outside during the day is valuable for everyone, whatever their latitude.
Realigning with the sky
Putting this into practice does not call for a complicated programme. Spending 10 to 30 minutes outdoors at the start of the day, having breakfast by a window, and taking short outdoor breaks during the day all make your internal clock's job easier. Shift work, long-haul travel and dark seasonal stretches can strain the clock; if sleep problems become persistent, it is sensible to consult a doctor. The content here is for information; you can find our approach to health content on the Health Content Policy page.
Checklist
- Try to get 10 to 30 minutes of daylight within an hour of waking
- Spend at least one break of the day outdoors
- Soften household lighting in the evening
- Cut down bright screen use in the hour before bed
- Note your sleep times through the week and watch how they change
Frequently asked questions
Do I need to look directly at the Sun?
No, certainly not, and looking directly at the Sun damages the eyes. What the internal clock needs is for the general brightness of the outdoors to reach the eye indirectly.
Does light through a window do the same job?
Partly. Sitting by a window is far better than being deep inside a room; but glass filters out part of the light, and outdoor brightness levels are usually not reached.
What can be done in winter and on dark mornings?
Getting outside at the first opportunity after sunrise and spending daytime breaks in the open air both help. Those with marked symptoms of seasonal low mood should consult a specialist.
Is a night-time screen filter enough?
Colour filters reduce only one component of the light; brightness and the stimulation of the content remain. A filter is not a solution on its own but a small support alongside reducing screen time and brightness.
This guide draws on peer-reviewed research findings in sleep and chronobiology and on the publicly available information of official health bodies.