▁an ▁experimental ▁dis ruption ▁of ▁the ▁fish ' s ▁normal ▁rest ▁behaviour ▁at ▁night . ▁ ▁Abs ence ▁of ▁sleep ▁▁ ▁Many ▁pel ag ic ▁fish ▁species , ▁such ▁as ▁blue fish , ▁Atlantic ▁m acker el , ▁t una , ▁bon ito , ▁and ▁some ▁sh arks , ▁sw im ▁continu ously ▁and ▁do ▁not ▁show ▁signs , ▁behaviour al ▁or ▁otherwise , ▁of ▁sleep . ▁ ▁It ▁has ▁been ▁argued ▁that ▁one ▁function ▁of ▁sleep ▁is ▁to ▁allow ▁the ▁brain ▁to ▁cons olid ate ▁into ▁memory ▁the ▁things ▁it ▁has ▁learned ▁during ▁the ▁animal ' s ▁normal ▁period ▁of ▁activity . ▁The ▁brain ▁might ▁not ▁be ▁able ▁to ▁do ▁this ▁while ▁still ▁ass a iled ▁by ▁new ▁stim uli ▁and ▁new ▁information ▁to ▁process . ▁Therefore , ▁the ▁role ▁of ▁sleep ▁would ▁be ▁to ▁period ically ▁shut ▁down ▁sens ory ▁input ▁to ▁allow ▁the ▁brain ▁to ▁form ▁mem ories . ▁Pel ag ic ▁species ▁sw im ▁in ▁an ▁environment ▁that ▁is ▁rather ▁b oring ▁( kil om eters ▁upon ▁kilom eters ▁of ▁open ▁water ▁with ▁not ▁much ▁happening ▁in ▁it ). ▁In ▁such ▁species , ▁the ▁sens ory ▁input ▁is ▁so ▁low ▁that ▁memory ▁formation ▁could ▁take ▁place ▁even ▁if ▁the ▁fish ▁keeps ▁on ▁moving ▁( a ▁repet itive ▁activity ) ▁and ▁does ▁not ▁fall ▁as leep ▁in ▁the ▁traditional ▁sense ▁of ▁the ▁word . ▁ ▁Di urn al ▁dam self ish ▁normally ▁sleep ▁motion less ▁in ▁cre v ices ▁within ▁cor al ▁re ef s ▁at ▁night , ▁but ▁three