In birds, sleep consists of "periods of eye closure interrupted by short periods of eye-opening." During the short periods of eye-opening, electroencephalographic (EEG) studies indicate that the birds are still sleeping; the voltage level in the brain is identical. Birds restore their arousal thresholds during sleep. During their short eye-open periods, sleeping birds can mobilize almost instantaneously when threatened by a predator. Avian species have been found to rely on flock size and height for predatory precautions. Between the eye-opening and group sleep, these precautions allow sleep to be beneficial and safe.