f you're over 50, there's a good chance you keep a bottle of vitamin D in the cupboard. You take it for your bones, or a doctor told you your level ran low. Most older folks do come up short on it. And that little pill can cost you $10 to $30 a month, month after month.

Maybe you wake up tired even after eight hours in bed. Or your mood sags by mid-afternoon, and sleep comes hard at night. Your fix for all three may be waiting right outside your door.

The best source of vitamin D was never in a bottle. It's free, it's outside, and it lands on your skin every clear morning. Morning sun also sets your sleep clock and lifts a gray mood. So before you buy another supplement, step outside.

Here's how we know it works...

Back in 2002, a team of doctors in Melbourne, Australia set out to measure it. Dr. Gavin Lambert led the group at the Baker Heart Research Institute. Their method was bold. They drew blood straight from a vein in the neck in 101 healthy men. That vein was chosen for a reason — blood there runs almost straight from the brain. So it gave them an honest read on mood chemistry.

What did they find? On bright, sunny days, the men's brains made the most of one key mood chemical. Dark winter days brought the least. That chemical is serotonin — the one that keeps your mood up and steady. Sun raised it, and gloom pulled it down. That is why so many people feel low and heavy through the short, dim days of winter...

So how does light reach that deep into you? It starts at your eyes. Each morning, light lands on the back of the eye and pings a tiny clock in your brain. Scientists call it the master clock. This little clock reads the light and sets the rhythm of your whole day.

For sleep, the master clock does one great job. When morning light strikes it, a timer begins. About 14 to 16 hours later, your brain drips out melatonin — the hormone that makes you drowsy. So the sun you catch at 7 a.m. is what helps you drift off at night. Miss that morning light, and the timer starts late — which is why sleep comes hard.

The catch is that indoor light is weak. A bright room gives your eyes about 500 units of light. Step outside on a cloudy morning and you get 10,000 or more. Your master clock barely notices the lamp, but it reads the open sky loud and clear.

Worth knowing: The U.S. National Institutes of Health notes that sunlight is the main way most people get vitamin D. Very few foods carry much of it on their own.

Step outside in the morning and you collect three gifts at once. The light sets tonight's sleep. Get it early and you drift off sooner. It lifts your mood, the way those men in Melbourne showed. And the sun on your skin builds vitamin D — the same nutrient in that costly bottle, made fresh and free...

Some people call the sun nature's own mood lifter. There's real truth under that old line. The vitamin D you make also pulls calcium into your bones and keeps them firm as the years pass. One short walk feeds your sleep, your mood, and your bones — all before breakfast. That's a lot of good from a habit that costs nothing.

Food can't take the place of sun. But a few foods lend a hand. A serving of fatty fish like salmon can cover much of a day's vitamin D. Egg yolks hold a little too. And mushrooms bring a real surprise.

Mushrooms pull off a trick much like your skin. Set them in the sun, and they turn light into vitamin D. A few hours gill-side up on a bright sill can raise their level in a big way. The change is quick and needs no gadget. Then you cook them and eat the reward.

Your body also needs magnesium to switch vitamin D on. Leafy greens are loaded with it — kale, spinach, chard. A daily handful helps the sun's work take hold. Eat them with a drizzle of olive oil, since vitamin D rides on fat into your blood. Whole foods and sunshine pull in the very same direction.

Get outside within an hour of waking. Aim for 10 to 20 minutes. You don't need to look at the sun — just let it reach your face and eyes. Even a cloudy sky pours out far more light than your kitchen bulb.

Slice a handful of white or portobello mushrooms. Lay them gill-side up on a plate in direct sun for two to three hours. They will make vitamin D you can eat that night. Call it the cheapest supplement in your kitchen.

Pair the morning light with a short walk. Movement wakes the body while the sun wakes the clock. Fifteen minutes covers both at once. Keep the walk outdoors, not on a treadmill by a wall.

Those men in Melbourne gave blood from their necks to teach us one plain thing. Your brain runs on light. Feed it sun in the morning, and it pays you back all day — steadier mood, deeper sleep, stronger bones. None of it costs a dime.

You don't need a bottle to begin. No plan, no gear — just open the door and step into the light. Your body knows what to do with it.

Try it this week. Step out each morning while the sun sits low. See how you sleep seven days from now.