When yoga was first created hundreds of years ago, practitioners didn't have the benefit of modern science to explain why it worked—they just knew it did. Nowadays, we can look to science to help us understand how yoga gives us relief from stress.
What Happens When We Are Stressed
To talk about stress, we have to talk about the hormone cortisol. Under normal conditions, your brain slowly releases cortisol into your body to make your muscles work. However, if you're highly stressed (late for work, deadline due, screaming baby: take your pick), your brain thinks there's an emergency and responds by flooding your body with cortisol.
If there really was an emergency, you would need this much cortisol to deal with broken bones, excessive bleeding, and so on. But if you're just stressed out by life, excess cortisol can lead to ulcers, high blood pressure, bone density loss, and can throw your immune system out of whack (why we get sick when we're stressed). It can also lead to weight gain because the body thinks it's responding to an emergency and so holds onto as much fat as it can.
How to De-Stress
Your nervous system has several branches. One of these branches controls what your internal organs are doing, and it has two parts: the sympathetic, or "fight or flight" state, which ramps up when you are stressed (and dumps cortisol into your bloodstream).
The parasympathetic, or "rest and digest" state, which controls more day to day functions. So if we're trying to get less stressed, we'll need to harness that "rest and digest" state.
The only problem is these two states are happening automatically within our bodies and we can't control them.
However, yoga has an answer. The diaphragm, the muscle that we use to breathe, mostly functions automatically, but we can also control our breathing when we want to (like holding our breath underwater). If we practice specific breathing patterns (called pranayama in yoga) we can slip ourselves into that "rest and digest" state and allow our minds and bodies to get some relief.
A Simple Breathing Practice
A simple way to harness the breath is to lie down, either on the floor or in bed, and close the eyes. Imagine the inside of your body like a balloon, and as you inhale, watch the breath expand the low belly, then up into the ribs and chest.
As you exhale, let all the breath come out as it wants. Release naturally. Repeat this pattern for up to five minutes. If it's hard to feel the different places where the breath moves, try gently placing one hand on your belly and one on your chest.
You can practice this relaxing breath at night before going to sleep and see what a difference it makes.