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Mornings matter more than bedtime rituals
Light tells the body
Light is how the body learns it is time to be awake. It reduces melatonin, the sleepy feeling, and it is good at reducing fatigue. A steady get-up time and morning light both do more for sleep than an over-ritualistic bedtime routine.
The Harvard light study
At Harvard, sleep researchers deprived people of sleep and then exposed them to quite bright light. The effects of a sleep debt did not vanish, but results on psychometric tests improved when the light arrived at the right time of day.
Winter light
Winter makes enough light hard to get. Artificial light can fill in, although natural light is better and brings extras such as vitamin D. The pattern is more light through the first three-quarters of the day and less in the evening.
How does morning light reach the body clock?
In 2002, David Berson and colleagues at Brown University identified a class of light-sensing cells in the retina, the intrinsically photosensitive retinal ganglion cells. They contain a pigment called melanopsin that responds most strongly to blue light, around 480 nanometers. These cells send signals along a dedicated route called the retinohypothalamic tract to the master clock in the brain. They work even in people who cannot see because of rod and cone damage.
Why does the timing of light matter so much?
Charles Czeisler's group at Harvard mapped how the human body clock responds to light at different times of day. Light in the early morning pushes the clock earlier, while light in the evening and early night pushes it later. Light in the middle of the day has little effect on timing. This pattern is called a phase response curve, and nearly every studied species has one.
How much dimmer is indoor light than outdoors?
A typical lit office or living room measures about 100 to 500 lux. An overcast sky outdoors delivers roughly 1,000 to 10,000 lux, and direct sunlight can exceed 100,000. The gap is why a short time outside can deliver more light than a full day under indoor lamps. In high latitudes in winter, the sun may stay above the horizon for only a few hours.
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