Intensive care unit (ICU) environment is not propitious for restoring sleep. Alterations in sleep have potential detrimental consequences explaining increasing interest in the field over the last years. Methods to study sleep in ICU meets some limitations. Accurate sleep analysis requires full polysomnography, but polysomnographic patterns of normal sleep are frequently lacking in these patients and conventional scoring rules may be inadequate. Patients experience severe alterations of sleep with sleep loss, sleep fragmentation and sleep–wake cycle disorganization. Many factors may contribute to these abnormalities, including patient-related factors (e.g., disease severity) environmental factors (e.g., continuous exposure to light and noise, around-the-clock care, and medications). Health support techniques such as mechanical ventilation and sedation may also contribute to sleep disruption. The impact of sleep disturbances on morbidity and mortality in ICU patients remains unknown but inferences from experimental studies or indirect evidence suggest possible immune function alterations and neuropsychological dysfunction that could hamper weaning from assisted ventilation. Whether sleep disruption in ICU patients is independently associated with adverse outcomes or merely constitutes a marker for cerebral dysfunction remains to be determined. However, whatever signification and mechanisms of these alterations, now specific measures are recommended to protect sleep and circadian rhythm in ICU.