Pillow height, sleeping position, and what the research actually shows

Pillows are sold by firmness and by filling. The measurement that turns out to matter most is neither: it is height, and there is a figure for it.

Ten to twelve centimetres, with neck support

A study of nine volunteers testing pillows of 8, 10, 12 and 14 cm found that an individualised height of around 10 to 12 cm, with neck support, produced the cervical curve closest to that of standing upright. Nine volunteers is a small study and deserves to be read as such — but the principle it illustrates is not controversial: the pillow's job is to keep the neck in the alignment it holds when you are standing.

That also explains why the same pillow suits one person and not another. A broad-shouldered side sleeper needs more height than a slight back sleeper to reach the same alignment. Height is individual; the target — the standing curve — is not.

Position: the night is not spent in one

The idea of having a sleeping position is largely a fiction. A reference study filmed sleepers from five age groups over four consecutive nights and recorded 3.6 changes of position per hour among 18-to-24-year-olds, 2.7 among 35-to-45-year-olds and 2.1 among 65-to-80-year-olds.

Two things follow. Movement decreases markedly with age, and nobody holds a position for long. Bedding that only works in one posture will be working against its owner for most of the night.

The glymphatic claim, and what the study really says

The claim that sleeping on your side clears the brain better circulates widely, usually without its source. The original study does exist — Lee and colleagues, Journal of Neuroscience, 2015 — and it shows that glymphatic transport was most efficient in the lateral position.

It is an animal study. That does not make it worthless, and it does not make it a human recommendation either. It is the kind of finding worth knowing about and not worth reorganising a life around, and any page that presents it as settled advice for humans is overstating it.

One case where the recommendation is firm

Pregnancy is the exception where a position is actually advised. The British organisation Tommy's, drawing on the MiNESS study and six concordant pieces of work, advises falling asleep on the side from twenty-eight weeks. The wording matters: falling asleep on the side, not staying there all night, which nobody does.

This is also where a body-length or U-shaped pillow earns its place — not for comfort alone, but because it makes the advised position the easy one to fall asleep in.

What to check on the pillow you already own

Lie on your side and have someone look at you from behind: the neck should continue the line of the spine, neither dropping toward the mattress nor pushed up. Then do the same on your back. A pillow that is right in one position and wrong in the other is a pillow you will fight for part of every night.

Sources: Journal of Neuroscience, 2015 (animal study); Tommy's / MiNESS; pillow-height study, nine volunteers.