Recent work suggests that "Animals that live in groups do not sleep in isolation":

https://www.sciencedirect.com/science/article/pii/S016953472...

So now we just need to know what percent of animals live in groups.

While older studies suggest that most mammals live alone, new field data show that only 22% of mammals are solitary: https://zslpublications.onlinelibrary.wiley.com/doi/10.1111/...

Keep in mind that my claim doesn't require them all sleeping together - any protective any social behavior would suffice (e.g. Having look-outs, etc.)

Since it's such simple reality, surely you can support your claim?

Maybe you were talking about non-mammals?