Why do we only snore when we’re asleep? Why don’t people who snore wake themselves up with their snoring? – Sue

Ask any snorer why their sonorous rumblings don’t wake them up and they will almost inevitably give the same, simple response: “Why ask me? I don’t snore!”

A snorer’s blissful ignorance of their own sounds gives the impression that they must sleep soundly through them, while the rest of the household listens on in frustration or horror. But just because they don’t remember waking up doesn’t mean they sleep like a baby. To explain why, we need to look at why some of us snore in the first place. Let’s break it down.

Your mouth and throat are full of all sorts of soft, floppy bits, such as your uvula, tonsils, adenoids and other bits of tissue.

When we’re awake, your body holds all these bits in position, ready for action. But when we fall asleep, our muscles relax and everything is free to loosen up.

This relaxation is an important part of sleep. As well as allowing our bodies to rest and recuperate, partial muscle paralysis prevents us from acting out our dreams and walking. While a live action mime of our dreams could be an amusing insight to spectators, it could also be dangerous.

As well as keeping our limbs safely tucked in bed, sleep relaxation affects the muscles that hold everything in place. For some people, this relaxation is enough for the soft tissues in their mouths to flop into undesirable positions and partially block the flow of air as they breathe.

Snoring is the resulting sound of all the oral smooshy bits vibrating and slapping together as air forces its way through the obstruction when we breathe.

Evolution has set us up to be snorers

Those mouth parts that cause all the trouble are actually the result of human evolution. A perfect anti-snoring airway would be a long, straight tube with no soft parts at all. Unfortunately, a lot more is required of our airways than just unlaboured breathing. In order to vocalise beyond simple grunts, faces and throats have been shaped to

accommodate more sophisticated sound apparatus – most of which is soft tissue. Our tongues have migrated back into our throats to shape different sounds. Compared to other mammals, our tongue rests precariously close to the back of our upper airway – the perfect place to become a blockage when we snooze.

Our upright posture has also had an effect, shifting throats underneath skulls and leaving less room in which to fit all those squishy bits – prime conditions for the airway obstruction that leads to snoring.

Loud sounds wake us up. Why not snores?

A loud crash from the kitchen in the middle of the night is almost certain to wake us up. Whether it’s a tree falling or a pet’s overly ambitious adventure, human bodies react to the sound by snapping speedily into a state of awareness.

This is because our ears are still taking in sound while we’re asleep, and our brain is still processing . Brains prioritise restfulness while we sleep, filtering out low-priority sounds and letting us snooze through unimportant background noise.

Only high-priority signals trigger wakefulness: research has shown we’re more likely to respond to unusual sounds, especially loud sounds that could signal danger, and someone speaking our own names.

For the offending snorer, the brain interprets soft snores as innocuous background noise that needs no further attention. But what about the ones that rattle the roof shingles? In fact, very loud snores actually can wake the snorer, but only briefly. We usually need to be in a very deep sleep state for our muscles to be relaxed enough for snoring to start, and at that point our brains are shutting out all but the most important information.

Even if a snore is thunderous enough to make it through this filter, the snorer slips right back to sleep within seconds. Brainwave research suggests that we can have up to 25 of these “microarousals” per hour without even noticing.

Jamie Priest