At 2 A.M. on November 1, the clocks will fall back, and for one weekend out of the year, the whole country will think about light and time. Most people will spend the extra hour asleep and the following week complaining that it’s nearly dark at 5 P.M. Regardless of how you feel about the time change, for the next four months the sun will rise in the sky closer to the time you do. That alignment is the best thing that could happen to your body clock—and your sleep—all year.

Of course, you can try to buy your way into a better night’s sleep right now: mouth tape, tablets of magnesium glycinate, blackout curtains, a wearable sensor that grades your sleep, a mattress that maintains a chill 62-degree-Fahrenheit temperature. The stack of such approaches can run into the thousands of dollars and has a name on social media—“sleep maxxing.” It’s the wellness market’s latest way of turning a basic human function into a product line.

But the two things that will do the most for your sleep cost nothing. That is because nobody has figured out how to package them.

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I’m talking about light: bright days, dark nights. Think of it as a return to nature. Call it Paleo lighting.

I shamelessly borrowed the term from the “Paleo diet,” which sold the premise that human physiology took shape under conditions that no longer exist and that the mismatch costs us something. We built an industry around what goes into our mouths. We left the eyes alone. Our ancestors could not dim the day, postpone the night, or order light at midnight. That is not an argument for candlelight or for unplugging the grid. It is the same dietary logic aimed at the organ that sets our internal clock.

This clock sits at the base of the brain, about 40,000 nerve cells in a structure called the suprachiasmatic nucleus (SCN) in the hypothalamus, and it generally runs slightly long. Shut a person in a laboratory with no windows and no clocks, and typically the body settles into a day of roughly 24 hours and 12 minutes.

Light early in the morning is what pulls the clock back in line with the sun. Bright light at night does the opposite, telling it the day isn’t finished. And the clock in your head is not the only clock you own. Your liver, pancreas and immune cells all run on their own daily schedules, taking their cues from that cluster of cells in the hypothalamus linked to the sun. When the SCN loses alignment with the sun, these rhythms drift apart and optimal functioning may be lost.

The disruption to our circadian rhythms comes at a physiological price. Light at the wrong hour can suppress melatonin, the hormone that tells your body it is night. And from there, the effects may spread to digestion, heart rate and the timing of metabolic and immune programs evolved to run on a 24-hour schedule. How far that reach extends is still being mapped. The clearest view comes from animals, for which you can hold everything else steady and change only the light.

In my laboratory, we have kept mice at night with lights set to five lux, roughly the brightness of a night-light in a kid’s bedroom. They gained more weight than mice sleeping in genuine darkness. They were not sleep-deprived; the night-light just disrupted their clock-gene rhythms, subsequently shifting their food intake to the wrong part of the day. In the case of nocturnal mice, they shifted their food intake from night to daytime. Because they generally sleep during the day, light at night can scramble their circadian timing without touching their sleep or rest time. Related work found the same dim night worsened weight gain, glucose control and inflammation in animals on a high-fat diet. Sleep, metabolism and immunity seem to act as gears in a clock: if there’s one misalignment, the whole system is disrupted.

The obvious objection is fair: these are mice, they are nocturnal, they live in cages, and nobody has ever asked one how it slept. What happens when you run the experiment on people?

Researchers at Northwestern University put healthy young adults to bed for a single night in a room lit to 100 lux—dimmer than a well-lit kitchen, brighter than a night-light—and measured them in the morning. Their heart rates ran higher through the night. And by breakfast, they were measurably worse at handling sugar, assessed by increasing insulin resistance, than another group that slept in the dark.

That was one night in a laboratory, with 20 participants. “What happens over years in ordinary bedrooms?” is the harder question, and the best answer we have comes from 43,722 women, none of them shift workers, who were tracked for about five years. Those who slept with a television or a light on in the room were 17 percent more likely to gain 11 pounds or more and 33 percent more likely to develop obesity over that time, after researchers accounted for diet, exercise and sleep duration. Night-shift workers, who experience the extreme case of circadian rhythm disruption, carry higher rates of type 2 diabetes, cardiovascular disease, mood disorders and several cancers—though people who live that far out of phase with the sun differ from non-shift workers in many ways besides light.

The prescription costs almost nothing. Within an hour of waking, get outside for 10 to 30 minutes in whatever the sky is offering—an overcast morning outdoors still runs to thousands of lux; a well-lit kitchen only manages a few hundred. Sit near a window if you have one. In one small study, office workers with windows got far more daytime light than colleagues without them and slept the better part of an hour longer at night. When indoors in the evening, dim the lights and use warm light bulbs rather than overhead and blue lighting.

Then deal with the bedroom, which is the part everyone skips. Stand in yours tonight with the lights off and count what still glows: the router, the smoke detector, the charger, the standby light on the television, the sliver where the curtains refuse to meet.

Nobody is ever going to sell you a dark night. You will have to make it on your own.