▁Circ ad ian ▁rh yth ms ▁of ▁activity ▁have ▁been ▁documented ▁in ▁over ▁ 4 0 ▁different ▁fish ▁species , ▁including ▁h ag fish , ▁lamp rey , ▁sh arks , ▁cy pr in ids , ▁ ict al ur ids , ▁g ym not ids , ▁sal mon ids , ▁and ▁lab rid s . ▁ ▁Phys i ological ▁sleep ▁ ▁One ▁phys i ological ▁characteristic ▁of ▁sleep ▁goes ▁by ▁the ▁name ▁of ▁" home ost atic ▁reg ulation ". ▁This ▁is ▁the ▁notion ▁that ▁animals ▁need ▁a ▁more ▁or ▁less ▁constant ▁amount ▁of ▁sleep ▁every ▁day , ▁so ▁that ▁if ▁a ▁subject ▁is ▁dep riv ed ▁of ▁sleep ▁one ▁day , ▁the ▁amount ▁of ▁sleep ▁tends ▁to ▁" re bound " ▁( in cre ase ) ▁the ▁next ▁few ▁days . ▁This ▁has ▁been ▁observed ▁in ▁z eb raf ish . ▁At ▁night , ▁z eb raf ish ▁appear ▁to ▁float ▁in ▁the ▁water ▁column , ▁either ▁hor izont ally ▁or ▁with ▁the ▁head ▁slightly ▁up . ▁The ▁frequency ▁of ▁mouth ▁and ▁g ill ▁movement ▁is ▁reduced ▁by ▁almost ▁half ▁and ▁they ▁are ▁twice ▁as ▁hard ▁to ▁ar ouse ▁as ▁during ▁the ▁day . ▁If ▁they ▁are ▁dep riv ed ▁of ▁this ▁sleep - like ▁behaviour , ▁the ▁sleep ▁b outs ▁there after ▁are ▁longer ▁and ▁the ▁ar ous al ▁threshold ▁is ▁higher ▁than ▁usual , ▁suggesting ▁a ▁re bound ▁effect . ▁ ▁Similarly , ▁in ▁the ▁conv ict ▁c ich l id , ▁activity ▁decre ases ▁on ▁days ▁that ▁follow