How Sharing a Bed Changed My Sleep

In order for those measure to make any sense OP would need to follow the exact same routine with and without his girlfriend (in the same flat of course). Otherwise it is measuring the general effect of living with somebody (different activities, different morning arrangement, different commute, ...) rather than the specific sleeping arrangement effect.

Now I'm not discarding the effect, however after a few years together, you start getting used to your bed partner. The stuff that really matters is if you have enough room and sheets to make it work. In the extreme case, a queen size bed is basically 2 single bed joined together and is a cheaper alternative than the extra half a million an extra bedroom costs.

edit: reread the article and it seems OP actually talk more about the effect of living with somebody rather than the physical sleeping arrangement. Comments were drifting in the "physical" direction at the time I commented. I definitively share the same experience - obviously with a kid and wife, I have much less freedom in my night and morning routine. Not sure I'm sleeping better, my wife definitively is - her stress level is way lower when she knows I'm home and she sleeps more deeply, not woken up by every little noise (the effect has been mitigated slightly since we have a child though)

1) Starting cohabitating relationship: First a transition period where you lose sleep, but then you sleep like a baby, you don't stay up late thinking or worrying about x,y,z. The downside is you find yourself in a stagnating ball of comfort and end up challenging yourself a lot less because you're just too comfortable. (I look back at how lame I became in some of my cohabitating relationships, and could say the same for others I know, but would not to their face.)

b) Leaving cohabitating relationship: You sleep like a baby at first (nice surface feeling of independence), maybe for a couple months even, but then you grow anxious and feel negative effects of dependency withdrawal, and then end up losing a lot more sleep staying up late thinking or working. That can last a while, until you find another cohabitating relationship or find a deep rooted sense of independence again.

Again, likely different when you're older, but I think that's the general trajectory of being in cohabitating relationships in your younger years.

I still wonder how anybody could voluntarily opt for a second one.

Thank God for the hardwired unconditional love for one's own children (I have heard that occasionally it doesn't work — poor souls). And still, I never even remotely expected anything like what has happened.