Right now, one of your nostrils is doing most of the work. In a few hours, it will hand the job to the other one. Most people never notice, but it happens to the majority of us every day.

It’s called the nasal cycle. Inside your nose, tissue called the turbinates swells and shrinks in alternating patterns. When one side swells slightly, air flows mostly through the other side, and then they swap. German physician Richard Kayser first described it in 1895, and researchers believe most people have it. The total amount of air you breathe stays the same, which is why you rarely feel the switch. Your autonomic nervous system, the same system that controls your heartbeat and digestion, runs it quietly in the background.

For many people it flips every few hours, but the timing differs from person to person and even from day to day, so don’t worry if yours doesn’t follow a neat schedule.

Scientists don’t fully agree on why your body does this, but there are a few leading ideas. It may let each side of your nose rest and stay moist, since the side with more airflow dries out faster. It may help your sense of smell, because slower and faster airflow carry different scents to your sensors. It may also support your nose’s defenses against germs.

Some things make the switch easier to notice. Lying on your side usually makes the lower nostril feel more blocked, which is why you may roll over at night without thinking. Colds, allergies, and dry air make the swelling stronger, and exercise or stress can temporarily open or narrow both sides.

You can test it yourself. Gently close one nostril with a finger and breathe in, then switch sides. Try it now and again in a couple of hours, and chances are one side will feel clearly freer, then the other later.

You may also come across claims online that breathing through a certain nostril boosts energy or “balances the brain.” Those ideas come from traditional practices. Slow breathing does help many people relax, but there’s little solid evidence for nostril-specific effects.

A switching blockage is completely normal. But if one side stays blocked for weeks, or you notice pain, nosebleeds, or a weaker sense of smell, get it checked, since a deviated septum or nasal polyps can be behind it.