Many insomnia patients show a marked discrepancy between objectively measured sleep duration vs.  subjectively reported sleep duration. For example, a previous meta-analysis found that patients with insomnia sleep only 23 min less per night objectively than good sleepers, while self-report sleep diary data indicate a mean subjectively reported difference of almost two hours between groups. There is also a systematic underestimation of sleep of about an hour when comparing subjective and objective measures of a particular night of sleep in patients with insomnia.

A recent study used sleep diaries and objective sleep recordings in insomnia and good sleepers. The study found that insomnia patients underestimated objective sleep by just under an hour, while good sleepers overestimated objective sleep by about 20 minutes. These findings underscore the finding that insomnia patients are much less accurate in estimating actual sleep duration compared to good sleepers, and that insomnia patients underestimate their sleep by about one on average.