Why a good night's sleep starts with morning light

As any sleep expert knows, preparing for a great night’s sleep starts the moment you wake up – because the key ingredient is morning light.

If you’re struggling to fall asleep on time and then cannot get up in the morning, it could be a sign that your body clock is running late. 

Most advice for that problem is about the evening: fewer screens, dimmer lights, a wind-down routine. Those things stop the clock from drifting later, but outdoor light in the first hours of your day pulls your clock earlier – which enables you to fall asleep on time.

In this guide, we explain how timing decides the direction, and how much light it takes to move your clock.

Key takeaways

• Light before your body’s temperature low point pushes your clock later. Light after it pulls your clock earlier.
• In the study that mapped this, the same light produced a range of just over five hours of clock shift depending on when it arrived.
• Very little evening light is needed to delay your clock. Room lighting under 200 lux delayed melatonin onset in 99% of people.
• The exposures that reliably advanced clocks were in the thousands of lux, far brighter than indoor lighting.

The evidence for morning light

A 2003 study gave 21 people several hours of bright light at different times of day, then measured what had happened to their body clocks.

Light in the morning made them sleepy earlier the following evening. Light late at night made them sleepy later. The switchover fell in the last hours of the night, shortly before waking.

Timing was the only thing that varied between them, and the gap between the largest delay and the largest advance was about five hours.

But remember: there’s equal evidence that bright or blue light in the evening shifts your rhythm the other way. So the benefits of morning light need to be offset by dimming lights later on, and cutting blue light wavelengths from phones and devices in the hour before bed.

How much light you need

A study using wrist-worn sensors calibrated against a lux meter recorded ambient light across indoor and outdoor settings for 55 adults over two weeks.

| Where | Illuminance | | Direct sunlight, summer midday | about 100,000 lux | | Bright light used in body-clock studies | around 10,000 lux | | Overcast winter day, outdoors | about 1,000–3,000 lux (as low as a few hundred on very dark days) | | Ordinary indoor lighting | about 100–500 lux |

Indoor artificial light came out at roughly a thousandth of peak daylight. Your eyes adapt across that range, which is why a well-lit room feels bright – but indoor lighting does almost nothing for your circadian clock. 

Even winter outdoor light, which is far lower than the summer figure, is still far above anything indoors.

That table above also puts the laboratory conditions in perspective. Studies that showed that morning light moved circadian clocks used controlled lighting at around 10,000 lux to mimic outdoor light. This sounds like a lot, but it’s still a fraction of what you get by stepping outside on a clear day.

How to use morning light for better sleep

• Get outside within an hour or so of waking. 
• Go outside rather than sitting by a window. Glass cuts the intensity substantially.
• Go out on overcast days too. Clouds reduce outdoor light considerably, and it remains far brighter than a lit room.
• Skip sunglasses for that first exposure. Normal sun sense applies once the sun is higher.
• Repeat it. The clock responds to a signal it receives every day.
• Dim the lights in the evening. Evening light undoes the morning’s work .

Frequently asked questions

How long should I be outside in the morning?

Ten minutes is a sensible floor, and more is better. The studies producing the largest shifts used hours of bright light, so there is no practical upper limit within a normal day.

Does light through a window work?

Less well. Glass cuts the intensity, and levels next to a window are far below outdoor levels. It helps, and stepping outside helps considerably more.

What if it is dark when I wake up?

In winter that is a real constraint. A bright indoor light or a light box is the usual substitute, and getting outside once it is light still helps. A consistent wake time matters more when light is unavailable.

Can I get too much morning light?

Not for your body clock. Ordinary sun protection applies for your skin and eyes once the sun is higher, but there is no circadian downside to a bright morning.

References

1. Khalsa SB, Jewett ME, Cajochen C, Czeisler CA. A phase response curve to single bright light pulses in human subjects. Journal of Physiology 2003;549(3):945–952. PMID 12717008 . 21 subjects. Peak-to-trough amplitude 5.02 h.
2. Zeitzer JM, Dijk DJ, Kronauer R, Brown E, Czeisler C. Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression. Journal of Physiology 2000;526(3):695–702. PMID 10922269 . 23 volunteers. Evening light and phase delay.
3. Gooley JJ, Chamberlain K, Smith KA, et al. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. Journal of Clinical Endocrinology and Metabolism 2011;96(3):E463–472. PMID 21193540 . 116 participants.