There are many forms of hibernation, so not every animal that hibernates does so in the exact same way.
Some animals like bats, for example, sleep deeply and don’t wake up during hibernation; others such as marmots get up from time to time. Also they don’t sleep all the time, but wake up and then sleep again – they don’t stay in the same position, they might turn around and move arms and legs to be comfortable.
What they all have in common is that the hibernation initiates a very passive and reduced state. During the months or weeks of hibernation, the animals don’t eat, so their metabolism is reduced, their body temperature lowered, as is their heart and breathing rate. This is what makes hibernation different from normal sleep. It affects the whole body.
To wake up they need a lot of energy as the body fires up the “engines” again. If a hibernating animal is disturbed during hibernation and has to wake up often, it might die (it will starve during a hibernation phase as it does not have enough energy reserves to survive). The waking up process is also driven by hormones and can occur during hibernation if the animal gets disturbed or has to flee/move.