A bivy is a bivouac sack - a waterproof fabric shell designed to slip over a sleeping bag.
Some of the risks of sleeping in a bivy tents are: Bears, lions, and snakes and the like. Humans have a smell to animals and then is the discarded food. These things can be very appealing in terms of attracting the wild animals.
Mountaineers typically sleep in tents or bivy sacks at high altitudes, often using sleeping bags rated for cold temperatures to stay warm. They may set up camp at designated sites or find sheltered spots to protect against wind and weather. To maximize rest, they often follow a routine, including preparing meals, securing gear, and adjusting their sleep schedule to acclimatize to altitude. Good sleep is crucial for recovery and maintaining energy for the next day’s ascent.