A syndrome of daytime consequences of insomnia and objectively severe sleep disturbance, which is not perceived by the patients, has recently been described. The present study aimed at retrospectively analyzing data on a group of such asymptomatic insomniacs.Twenty-seven middle-aged target patients who presented with daytime symptoms but denied sleep problems underwent polysomnography (PSG) and filled out a morning questionnaire. For classifying objective insomnia, sleep onset latency or wake after sleep onset (WASO) of more than one hour was required. In addition to PSG, alpha sleep and sleep time, overestimation was calculated.Comorbidity, mostly multiple (organic, psychiatric, functional, and sleep), was present in all patients. All had severe sleep maintenance disturbance (WASO>100 min), whereas sleep onset was partly normal. Overestimation of sleep was >2.5 h. Alpha sleep was present in 44%, but not related to higher overestimation than non-alpha sleep.Non-perception of intermittent waking times is crucial for this syndrome. Possible causative factors are discussed. A specific insomnia treatment, carried out in 14 patients, was able to reduce daytime symptoms significantly. Thus, recognition of this syndrome is clinically relevant. Studies are needed to elucidate the nature of asymptomatic insomnia.