Virtually all light can affect sleep, but not all types of light have the same impact. Daylight, which in direct sunlight has up to 10,000 lux, a unit to measure illuminance, is far more intense than even bright office lighting, which rarely reaches about 500 lux. For this reason, daylight is a profound influence on sleep and circadian timing.

There can be important differences between types of artificial light as well. Some types have more illuminance and brightness. That said, even light that appears to have the same brightness may, in reality, have a different wavelength, changing how it’s perceived by the eye and brain.

For example, blue light has a short wavelength and is emitted by many LEDs. Studies have found that it has a significantly larger effect on melatonin and circadian rhythm than light with a longer wavelength. Many electronic devices, including cell phones, tablets, and laptops, emit blue light, and their extensive evening use can contribute to sleep problems.

Is it Best to Sleep in Pitch Darkness?

As a general rule, it is best to sleep in as much darkness as possible. Pitch darkness reduces potential distractions and disruptions to sleep.

Sleeping with a light on interferes with sleep cycles and causes more fragmented sleep, and these downsides may be greatest in the few hours before waking up.

Research demonstrates that closing your eyes isn’t enough; your eyelids can’t block sufficient light. The effects on circadian rhythm can occur even with low levels of indoor light andclosed eyes.

In addition to sleep quality, there are other reasons why research indicates that it’s best to sleep in pitch darkness:

• Eye strain: Even low levels of ambient light during sleep have been associated with eye strain, resulting in soreness, tiredness, and discomfort in the eyes, as well as greater difficulty focusing.
• Weight gain: Keeping lights on during sleep appears to affect circadian regulation of metabolism, increasing the risk of weight gain even if sleep itself is not disrupted. In one study over a five year period, women who slept with a light or TV on were considerably more likely to gain 10 pounds or more, even after controlling for factors relating to their diet and exercise habits.
• Cancer risk: One observational study found an association between people whose homes had high levels of artificial light at night and their risk of breast and prostate cancers. This study did not demonstrate causation, and further research is necessary to understand this correlation.

The diverse potential consequences of excess artificial light in your bedroom reflect that light exposure may throw off circadian rhythm and its vital role in promoting numerous elements of physical and mental health.

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