I have two cats and most days at least one cat sleeps in my bed at night. So here are some observations and research conclusions:
- If the cat is shy/new letting the cat in the bedroom could help it to get used to the owner since many cats find humans that lie down less scary.
- If you are away from home for work during the day the cat may enjoy several hours during the night being close to you.
- If the cat wakes up in the morning before you get up it might be bored or hungry. Some cats would start scratching on the bedroom door at that point. You could/should train them to stay patient but it is probably nicer for them (and could be nicer for you too) to just join you in bed.
- Cats can act like a small heater in bed, which is very comfortable during winter.
- Waking up in the morning and seeing your cat curled up on your bed, petting it while slowly waking up, is a gentle start in the morning and feels very good for the soul.
- If you are sensitive or allergic, having animals in your bed is not a good idea.
- If you are pregnant you should be careful about close contact with animals. Cats, especially outdoor cats, can transmit a disease that can seriously harm the unborn baby if the mother gets a first-time infection during pregnancy.
- You never know what happens between a baby and a cat while you are asleep, it is not save to let cats sleep in your bed if you have a baby there sleeping with you.
- If the cats are used to sleeping in the bedroom and a reason comes up to shut them out (e.g. a newborn baby) it can be hard for them to get used to the new restriction.
- Some cats wake up their owners during the night or in the morning when they are hungry. Although behaviour can be trained, some cats might not learn or the owner might not manage to teach them. Sleep deprivation is unhealthy.
- If the relationship between the owner and the cat is not good or if the cat is old (cats can get dementia) or sick it could be in pain or disoriented and scratch or bite the owner during the night.
I think for domestic, tamed, healthy cats it is more likely that they would get seriously hurt by the owner during the night than the other way around. However, (grown up) cats are not so fragile and if they got kicked out of the bed by an owner's nightmare they might simply decide to sleep somewhere else (with their hurt ego). (Assuming that the bed is on level ground, bunk beds might be a different story...)