 Amphibians have periods of inactivity but show high vigilance (receptivity to potentially threatening stimuli) in this state.There are significant similarities between sleep in birds and sleep in mammals, which is one of the reasons for the idea that sleep in higher animals with its division into REM and NREM sleep has evolved together with warm-bloodedness.Birds compensate for sleep loss in a manner similar to mammals, by deeper or more intense SWS (slow-wave sleep).