How Long Should You Wait Between Screen Light Exposure and Bedtime?

Blue Light from Electronics May Induce Insomnia in Susceptible People

Using Technology At Night in Bed
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There is some concern that light exposure at night may affect sleep. What is the evidence that artificial light from screens contributes to insomnia? How long should you wait between screen light exposure and going to bed? Explore this relationship and some of the science behind the rationale.

The Science of Sleep

When considering how optimal sleep occurs, it is important to understand the two primary contributors: homeostatic sleep drive and circadian rhythm.

Sleep drive is the fact that the longer you stay awake, the sleepier you will become. This is due to a chemical that gradually builds in the brain with prolonged wakefulness. This chemical is called adenosine. High levels of adenosine contribute to the onset of sleep. Sleep, at least in part, is a process of clearing this chemical away until consciousness is restored. Interestingly, caffeine blocks this signal and alcohol enhances it.

The circadian rhythm is the complement to this system. It is predominately an alerting signal that strengthens during the daytime and is largely absent overnight. It becomes strongest in the late evening hours, when we would expect to feel fairly sleepy. There is a lull in the mid-afternoon, which can contribute to a desire to take a nap then.  The circadian rhythm is based in our genetics, persisting without external influences. It contributes to sleep-wake propensity, hormone release, and metabolism.

These processes are linked to the day-night cycle of light and darkness via the eyes.

How Light Changes Sleep

A simple anatomy lesson reveals the importance of light exposure to sleep. The eyes perceive light via the retina and pass this information along the optic nerves, extensions of the brain itself.

These optic nerves receive input from each eye and cross at a location called the optic chiasm. Just above this is an area of the brain called the hypothalamus, a functional area that is integral to the control of the processes described above.

Within the hypothalamus lies the suprachiasmatic nucleus. This is the control center of the body’s circadian rhythm. It is the central pacemaker, coordinating the activities of all the body’s organs, tissues, and cells. Therefore, light input can be directly linked to the influence of many of the body’s processes.

In particular, light exposure can suppress the desire for sleep. Morning sunlight exposure may help to wake us, initiating the circadian alerting signal. In the same way, artificial light at night may affect the timing of sleep. It may contribute to insomnia in susceptible individuals, especially among those with a tendency towards being a night owl (called delayed sleep phase syndrome).

Preserving Darkness in the Night

What impact did Thomas Edison have on sleep in inventing the light bulb?

As society has evolved, the potential disturbances have only expanded. With electricity, our evenings are filled with activity: televisions, computers, tablets, e-books, and phones that flicker light into our eyes. Moreover, the activity itself may keep us awake, shorten our total sleep time, and interfere with a relaxing buffer zone before bedtime. There is some evidence that light at night may adversely affect sleep.

Research has demonstrated that in the spectrum that we perceive as light, the blue wavelength is the one that can change our circadian rhythms. Therefore, industries have developed to supply filters and glasses that block the blue light. It seems that even short bursts of light, especially when of higher intensity, can impact sleep’s timing.

The sun is much more powerful in intensity compared to light from bulbs or screens. Full sunlight may be 100,000 lux in intensity while overhead lights may be just 1,000 lux. Therefore, a blast of sunlight at night could profoundly impact sleep. For susceptible individuals, artificial light may also negatively impact sleep onset and contribute to insomnia.

As a result, it is recommended that people who have difficulty falling asleep may benefit from reduced light exposure prior to bedtime. (This is especially true if you find you can't fall asleep until 2 A.M. and prefer to sleep in.) Preserve the last 1 hour, and perhaps longer in highly sensitive individuals, as an electronics-free zone. Power off your screens, especially those that are closer to your eyes. It is also advisable to keep your bedroom free of electronics to preserve sleep. This is particularly important advice for teens and children.

Why might some people not be bothered by light at night? The complementary system, characterized by sleep drive and building levels of adenosine, could counteract these effects. If you are sleepy enough, you may not be bothered by the light. If you struggle with persisting insomnia, consider speaking with a sleep specialist to help determine ways for you to improve your sleep.


Kryger, MH et al. "Principles and Practice of Sleep Medicine." Elsevier, 5th edition.

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