Chances are, everything feels a little different when you’ve been in the same relationship for a long time. And it turns out scientists have some evidence to back that up.

The research is a little difficult to pin down. Like Tolstoy, scientists have spent more time studying unhappy relationships than happy ones, which means there’s less out there than you might expect.

Plus, everyone — including scientists — defines a long-term relationship differently. Researchers think about couples in many ways: married, cohabiting, together 20 years, together 50 years, madly in love. That means some of the findings included here may apply to only one particular subset of people in “long-term” relationships.

But here are a few of the most intriguing observations scientists have made.

Your sleep is tied to your relationship’s health.

Perhaps unsurprisingly, sharing a bed with someone means that person can affect your sleep and vice versa.

Scientists are still working on figuring out exactly how, but they have a few early results.

Both men and women took longer to fall asleep when their partners were anxious. One very small study found evidence that men who didn’t sleep as well were grumpier with their partners the next day. And a third team has discovered happily married women tend to sleep better than those in unhappy relationships.