We sleep less well when we sleep together:

Our collective weariness is the subject of several new books, some by professionals who study sleep, others by amateurs who are short of it. David K. Randall’s “Dreamland: Adventures in the Strange Science of Sleep” belongs to the latter category. It’s a good book to pick up during a bout of insomnia. …

Research studies consistently find … that adults “sleep better when given their own bed.” One such study monitored couples over a span of several nights. Half of these nights they spent in one bed and the other half in separate rooms. When the subjects woke, they tended to say that they’d slept better when they’d been together. In fact, on average they’d spent thirty minutes more a night in the deeper stages of sleep when they were apart. (more)

In 2001, the National Sleep Foundation reported that 12% of American couples slept apart with that number rising to 23% in 2005. … Couples experience up to 50% more sleep disturbances when sleeping with their spouse. (more)

Why do we choose to sleep together, and claim that we sleep better that way, when in fact we sleep worse? This seems an obvious example of signaling aided by self-deception. It looks bad to your spouse to want to sleep apart. In the recent movie Hope Springs, sleeping apart is seen as a big sign of an unhealthy relation; most of us have internalized this association. So to be able to send the right sincere signal, we deceive ourselves into thinking we sleep better.