Meditators regularly misjudge how long they have been sitting, sometimes by a wide margin in either direction. The distortion is a straightforward consequence of how duration is estimated.
Duration is reconstructed rather than measured
There is no dedicated organ for sensing elapsed time, so estimates are assembled from other information, chiefly the amount of change registered during the interval.
An interval containing many distinguishable events is judged longer than one containing few, even when both occupy the same clock time.
This is why a varied afternoon and a monotonous one of equal length are recalled so differently afterwards.
Attention to time lengthens it
Monitoring how much time has passed adds a stream of time-related judgements, each of which contributes to the estimate.
A session in which somebody repeatedly wonders how long remains will therefore feel considerably longer than one in which the question does not arise.
The effect is the same one that makes waiting unpleasant, and it explains why timers are recommended rather than clock-watching.
Absorption compresses the estimate
When attention is fully occupied and few separate events are registered, there is little material from which to construct a duration.
Sessions of that kind are commonly reported as much shorter than they were, and the surprise on hearing the bell is a frequent description.
The same compression appears in absorbing work and in conversation, so it is not specific to meditation, though sitting makes it easier to notice.
Discomfort expands it
Physical discomfort produces a continuous stream of salient sensations, each of which supplies material for the estimate.
A session in which the knees hurt will therefore feel long, independently of anything happening mentally.
This is part of why posture receives so much attention in instruction, since an unsustainable position affects the whole experience of a sitting.
It also explains why the same person can find a twenty minute session unbearable on a hard floor and unremarkable in a chair, with no difference in what they were practising.
The distortion is data rather than noise
Because the estimate tracks how much has been registered, noticing that a session felt long or short reports something about the quality of attention during it.
Experienced practitioners use this, treating a marked misjudgement of duration as information about what the session contained rather than as a curiosity.
It also explains why sessions do not become uniformly shorter with practice, since a settled sitting and a restless one produce opposite distortions.