We studied sleep patterns for three nights in 10 subjects with moderate to severe progressive supranuclear palsy and correlated the findings with disease severity using quantitative measures of motor, cognitive, and eye movement impairment. All subjects had severe insomnia, spending 2 to 6 hours awake per night; the mean time awake per night for the group was more than 4 hours. Sleep latency became shorter and the number of awakenings increased with greater motor impairment, and total sleep time declined as dementia worsened. These findings indicate that in progressive supranuclear palsy insomnia is related to disease severity. Insomnia associated with progressive supranuclear palsy appears to be worse than the insomnia of Parkinson's disease or Alzheimer's disease and may be due to degenerative changes in brain structures responsible for sleep maintenance.