Six genetically narcoleptic dogs, as well as 6 age- and breed-matched control dogs, were recorded continuously for 24 h to compare sleep/wake patterns and to determine whether narcoleptic dogs exhibit evidence of excessive sleepiness. Compared with controls, the affected animals showed a substantial reduction in wakefulness and a significant increase in time spent in the drowsy state. Total NREM sleep and total sleep time (NREM and REM sleep) in the 2 groups, however, were very similar. Results suggest that, whereas genetically narcoleptic dogs show less wakefulness than controls, they are not hypersomnolent. However, the greater amounts of drowsiness in the affected animals suggest that they are 'sleepier' than controls. These results are highly consonant with those found in human narcolepsy.