Life is full of mysteries: What is dark matter? What happened to DB Cooper? And the most perplexing to me personally: how tall should pillows be?

I can never seem to find the right pillow. They always seem too soft and flat, or too tall and stiff, and I end up tossing and turning all night. Currently, there are six pillows on my bed, none of which are quite right.

Many people end up choosing the wrong pillow because they’re more focused on the qualities of the pillow than on how they sleep, says Dr Geet Paul, director of interventional pain medicine at the George Washington University School of Medicine.

“A pillow should keep the head and neck aligned with the spine,” says Paul. “But many pick one just because it feels soft or fluffy.”

Finding the proper pillow is about more than just comfort. The wrong pillow can disrupt your sleep, and “poor sleep quality is associated with poorer mental health, cardiovascular health, learning and memory function”, says Dr Doug Cary, musculoskeletal physiotherapy specialist and sleep educator.

Here’s how to pick the right pillow for your needs.

First, what type of sleeper are you?

Finding the best type of pillow for you depends on how you sleep.

There are four main sleep postures, says Cary: supine (on the back), prone (on the front), supportive side-lying (on the side with top knee behind bottom knee), and provocative side-lying (on the side with top knee forward of bottom knee).

Supine and supportive side-lying are considered “supportive postures”, and prone and provocative side-lying are considered “provocative postures”, he says.

“People who sleep in these provocative sleep postures were more likely to report a lower quality of sleep,” as well as waking spinal pain and stiffness, Cary says.

What’s the best pillow set-up for each type of sleeper?

When your head is resting on a pillow at night, your spine should be straight and well supported.

“Poor positioning – such as twisting the neck or arching the lower back – can strain spinal structures and contribute to discomfort or long-term issues,” Paul says.

To select the best pillow, you need to know which position you favor. You may be mostly a side sleeper, for example. Note that “no one posture will be sustained during the night,” Cary says. “Even ‘normal’ sleepers will change their sleep posture 15-20 times on average.” That number could be twice as high for “poor” sleepers, he says.

When it comes to materials, go with what feels best to you: memory foam, latex or regular foam. Fancy down pillows may be luxurious, but Cary says research has shown feather pillows to be the least comfortable.

Other factors to consider are mattress firmness and body weight. With a softer mattress, your body will sink deeper. Therefore, you will need less pillow height, Cary says. The same goes for higher body weights. And to maximally support the spine, it may be helpful to have more than one pillow.

Each sleep posture requires a different set-up, experts say.

For back sleepers: In terms of alignment, the most important thing “is making sure the chin is not touching the neck, which can happen as a result of a tall/firm pillow”, says Dr Lindsay Orosz, director of research at the National Spine Health Foundation. Find a pillow soft enough to leave “at least four fingers between your chin and chest”, she says.

Orosz also suggests back sleepers add support under the knees, like another pillow or a rolled-up blanket. This can reduce the anterior pelvic tilt – the arch under the back when a person lies down – and ease pressure on the lower back.

For side sleepers: The pillow needs to be high enough to keep your neck straight, but not so high that it forces the neck too far to one side, bringing the ear close to the shoulder. “A medium-firm pillow usually works best,” says Paul.

Experts also suggest putting a pillow between your knees to keep the hips and back in line. A good rule of thumb is drawing an imaginary straight line “from the tip of the nose, to the sternum, to the belly button”, says Orosz.

For front sleepers: “We would generally recommend trying not to sleep on your stomach,” says Paul.

According to the Cleveland Clinic, sleeping on your stomach can lead to back, neck and shoulder pain because people tend to twist their necks and tuck their arms under pillows.

If that is the position in which you most naturally sleep, Paul recommends placing thin pillows under the head and hips to help stabilize the spine.

How often should you change your pillows?

A pillow is not a lifetime investment.

“Most pillows need to be replaced every 1-2 years,” says Orosz. If a pillow gets lumpy, you can’t get comfortable on it any more, or you start waking up with neck pain, it might be time to retire it, says Orosz. Test this by bending your pillow in half, she suggests. If it regains its shape when you let go, it’s fine to use. If not, consider a replacement.

What other factors can cause discomfort?

If you have persistent neck and back pain throughout the day, your problem might not be the pillow. A different sleep issue or a more serious underlying issue might be to blame.

“If I am in a clinical consultation with a client who is expressing ongoing neck pain/stiffness and they have tried ‘every pillow under the sun’, this indicates that the pillow isn’t the source of their problems,” he says. He works with these clients on their sleep systems as a whole, including sleep posture.

And, to reiterate, don’t sleep on your stomach if you can avoid it.

“My strong clinical opinion is to avoid prone sleeping if you are serious about spinal health,” says Cary.