Why 10 Minutes of Morning Sunlight Changes Your Sleep at Night

When we can’t fall asleep at night, we usually start by fixing our evening habits. We put the phone down in bed, cut back on coffee, and go to bed earlier. All of that matters, but brain science recommends one more thing: the start of tonight’s sleep begins to be set the morning before. The key is the light that enters your eyes in the morning.

Your brain has a clock that runs a little longer than 24 hours

Deep in the brain’s hypothalamus sits a cluster of neurons smaller than a grain of rice: the suprachiasmatic nucleus (SCN). It is the master clock that conducts the daily rhythm of the whole body. Body temperature, hormone release, and the waves of sleepiness and alertness all follow its beat.

Interestingly, this clock does not run on exactly 24 hours. In experiments that removed every external time cue, the period of the human body clock averaged about 24.2 hours (Czeisler et al., 1999). If a clock that runs a few minutes slow each day is left alone, the times you fall asleep and wake up drift gradually later. So the brain has to reset its clock to the outside world every day, and the most powerful of those reference signals is light.

Your eyes have cells that tell time, not just cells that see

Besides the cones and rods we use to see objects, the retina contains special ganglion cells (ipRGCs) with a light-sensitive pigment called melanopsin. Instead of seeing shapes, these cells measure how bright the surroundings are, and they send that information straight to the suprachiasmatic nucleus (Berson et al., 2002).

With this signal, the suprachiasmatic nucleus confirms that “it is morning now” and moves the clock forward. As a result, melatonin, the hormone that brings on sleepiness, starts to be released on time in the evening. Conversely, when morning light is scarce and we look at bright light late into the night, the clock keeps drifting later. Research has shown that simply being exposed to ordinary room lighting before bed delays the onset of melatonin release and shortens how long it lasts (Gooley et al., 2011).

Indoor light is dimmer than you think

Because our eyes adapt quickly to differences in brightness, it is easy to feel that a room is bright enough. Measure it with a light meter, though, and the gap is huge. Typical indoor lighting is on the order of a few hundred lux, while even a cloudy day outdoors is usually several to dozens of times brighter, and clear sunlight reaches tens of thousands of lux. To your body clock, sitting by a window and stepping outside are completely different signals.

The time of day changes what the signal means

Light has opposite effects depending on when you see it. In general, light in the morning moves the clock earlier so you feel sleepy earlier at night, while bright light late at night delays the clock and pushes back the time you fall asleep. That is why “bright in the morning, dark at night” is more accurate advice than simply “get more light.”

Today’s practice

1. Go outside within an hour of waking. Walking a little farther on your commute, or drinking your coffee on the balcony or by the front door, is enough. On clear days aim for about 10 minutes; on cloudy days, a little longer.
2. Don’t look directly at the sun. What matters is the brightness around you entering your eyes; there is no need to look at the sun at all. Take off your sunglasses for this short time (if you have an eye condition, follow your eye doctor’s advice).
3. Run your lighting the opposite way in the evening. One to two hours before bed, switch from bright ceiling lights to a low lamp, and turn down your screen brightness.
4. Keep your wake-up time steady, even on weekends. The clock stabilizes when the morning light signal arrives at about the same time every day.

Light is a signal, not a medicine. Rather than expecting a dramatic change in a day or two, try getting morning light at the same time for about a week and note how your bedtime shifts. If insomnia lasts more than a few weeks, it is a good idea to see a sleep specialist.

References

• Czeisler, C. A. et al. (1999). Stability, precision, and near-24-hour period of the human circadian pacemaker. Science, 284(5423), 2177–2181.
• Berson, D. M., Dunn, F. A., & Takao, M. (2002). Phototransduction by retinal ganglion cells that set the circadian clock. Science, 295(5557), 1070–1073.
• Gooley, J. J. et al. (2011). Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. The Journal of Clinical Endocrinology & Metabolism, 96(3), E463–E472.