Couples who share a bed almost always have a system for how they share the bed. That makes sleeping together easier and more efficient. With things so well organized they don't have to spend time discussing how to use the bed tonight. An important part of the predictable system couples have is that they usually know which of them is going to sleep on which side of the bed. That was true for almost every couple I interviewed for my book on couple bed sharing (Two in a Bed). And for each of those couples there were reasons why they slept where they slept.
It's the Side I've Always Slept on
Some people establish their favorite sleeping spot before the partner comes along. Particularly if the couple is living where one of them lived before they were a couple, the partner who was there first usually keeps on sleeping on her or his preferred side of the bed. Sometimes the preference for side of the bed was even established in childhood, for example, growing up always sleeping against a wall.
I Can't Stand Feeling Trapped
Some people need to feel like they can leave the bed quickly if they have to. It's an issue for them of feeling safe and not trapped. So for them, sleeping against a wall or far from the door is uncomfortable. They want to be on side of the bed away from the wall and close to the door.
I Have to Get Up First
It's convenient for both partners if the person who is going to get going first in the morning is on the side of the bed close to the alarm clock and away from a wall. That way, the first person up can wake up and get out of bed without bothering the partner much.
I Sleep Closer to the Door to Protect Her
For some heterosexual couples I interviewed the man slept closer to the door so that if danger came into the bedroom, he would be the one to take on the danger. Even some men who felt that they would be instantly neutralized by a harm-doer entering the bedroom still slept on the side of the bed closer to the door as symbolic protectors.
I Need to Be Closer to the Bathroom
In some couples, because one partner makes more bathroom trips, that is the person who has the side of the bed closer to the bathroom. With easier and quicker access to the bathroom, the person can get in and out of the bathroom quicker and will do less on each bathroom trip to disturb the partner.
I'm Closer to the Door Because of the Kids
In couples with young children, the partner who would be most likely to want to respond to kids or need to respond to kids or who the kids would be most likely to go to usually sleeps closer to where the kids are. In heterosexual couples that is usually the mother, but sometimes it's the father, especially if he wakes up more easily and is more quickly functional after waking up.
I Need to Sleep on This Side Because of My Injury
Some people sleep on the side of the bed they sleep on because they have physical problems that make one side easier to get in and out of or more comfortable to lie on. For example, if they prefer to sleep facing out from the middle of the bed and their right shoulder hurts, they would sleep so that they could lie on their left side and facing out from the center of the bed.
I Prefer the Warmer (Cooler) Side
In some bedrooms one side of the bed is clearly warmer or cooler seasonally or year around, and that can affect who sleeps on which side of the bed. The partner who most needs to be warmer or cooler sleeps on the side of the bed that will be most comfortable for her or him.
I Need to Be Closer to the Bedside Table
If there's only one bedside table, who sleeps near or far from it has to do with who will use it in what way. For example, the partner who wants to stay up late reading would want to be closer to a lamp on the bedside table. Or the partner whose job has the potential for middle-of-the-night emergency phone calls might want to sleep closer to the table because that is where the cell phone or land line is.
He's Not Gonna Step Out on Me During the Night
One woman wanted her husband to sleep close to the wall because that would make it harder for him to get up in the middle of the night for a romantic encounter with another woman or a night of heavy drinking. He'd have to get by his wife to get out of bed, and his trying to would wake her up, and then she'd insist that he stay in bed.
Choice of Sides of the Bed When Sleeping Elsewhere
The couple bedside system is still in operation when a couple goes to a hotel or motel or pays an overnight visit to friends or relatives. Then, depending on their underlying system, a couple might reverse their positions, with the one who usually sleeps on the left sleeping on the right and vice versa. This is so if what is important to them is not who sleeps on the left or right but who is closer to the door, the wall (if there is one), the bedside table, the bathroom, or the children. The bottom line is that most couples have their pattern of sleeping on the sides of the bed they sleep on and have their reasons for doing things the way they do them.