Delta waves and deep sleep

Roughly 0.5 to 4 Hz. High amplitude, low frequency: the slowest and largest of the conventional bands.

Where they appear

Delta dominates slow-wave sleep, the deepest non-REM stage. Clinical scoring defines that stage by delta: when slow waves occupy a sufficient proportion of an epoch, the epoch is scored as deep sleep.

Slow-wave sleep is concentrated in the first half of the night. This is the mirror image of REM, which builds towards morning, and it is why the two halves of a night are not interchangeable.

Why it matters

• Waking from it feels terrible. Sleep inertia is at its worst out of slow-wave sleep: grogginess, disorientation, several minutes before you are usefully awake. A nap that runs long enough to reach it produces exactly this.
• It is what gets prioritised after sleep loss. Cut your sleep short and the following night recovers slow-wave sleep first.
• Dream reports from it are sparse and thin. Not absent, non-REM dreaming is real, but short, thought-like and rarely narrative.

What to be sceptical of

There is a substantial market in devices and audio tracks promising to "boost delta". Delta is a description of a waveform, not a resource. The interventions with actual evidence behind deep sleep are the dull ones: enough time in bed, consistent timing, and not drinking alcohol before it.