The longest and deepest sleep I experience is hibernation, which I do for 7 months over winter. Before that I spend time making myself a lovely comfortable nest and eat so much food I DOUBLE in weight. I also develop a thicker fur to keep me extra snug. Once I am all fat, warm and cosy I fall asleep and it takes a beautiful Spring day to wake me.
We hardly know anything at all about how sharks sleep yet. Some sharks need to ‘sleep’ while swimming because they are ‘ram ventilators’ that need to force water past their gills by swimming. Other species like catsharks can repose on the bottom of the sea and use a buccal pump to breathe even while ‘sleeping’. How they prepare their bodies to sleep and what is happening with their brains is almost completely unknown…we need some more shark biologists to figure this out!