My wife has an extremely difficult time going back to sleep if she is fully wakened. Our kids co-slept. She could feed them at night without having to get up, and still be able to mostly get some sleep.

As an additional benefit, she didn't have to pump for overnight, and I didn't have to get up to bottle feed. Which, I would have done! But I'm happy not to have to have done that.

I am a very hard sleeper. I can sleep through anything. But I can sleep in bed with a little kid next to me just fine. I think our unconscious minds/nervous systems must have enough awareness of those little blazing furnaces.