Hypnogram

The chart comes from the sleep lab. In polysomnography a technician or an algorithm scores the night in 30 second epochs, assigns each one a stage, and the hypnogram is simply that list of scores drawn as a line over time. Every consumer sleep app that shows you coloured bands across the night is borrowing the same picture.

Reading one is mostly about shape. A normal adult night drops quickly towards deep sleep in the first cycle, with the deepest, longest slow wave block near the start. As the night goes on the line spends less time at the bottom and more time in the upper bands, and the REM sleep blocks get visibly wider until the last one before morning is the widest. Short spikes up to wake are ordinary. A night with no interruptions at all would be the unusual one.

What the chart cannot tell you

A hypnogram flatters precision. It draws a hard edge at every stage change, when in reality the transition is gradual and the 30 second grid is a convention, not a fact of biology. Two scorers looking at the same recording will not agree on every epoch. So the useful questions are broad ones: did deep sleep land early, did the night hold together, how many times did it break, and how does the overall shape compare with your own usual.

That matters more when the stages are estimates. Tock draws a hypnogram in the night detail, and on a phone only night the stages behind it come from actigraphy and sound picked up by the microphone on your nightstand, not from brain activity. They are estimates, and they are honest about being estimates. Measured stages need Apple Health or a wearable, which Tock will read if you have one.

For what a phone can and cannot see overnight, sleep tracking without a wearable covers the mechanics, and sleep architecture is the term for the pattern the chart is drawing.