Side of bed debate - Which side is left?

Which side of the bed is the left side? Is the answer based on the perspective of laying in the bed (person's head at the head end)? Is the answer based on viewing it from the foot of the bed, looking at the head of the bed? Is there an "anatomical position" or special terminology like in boating for this?

For context: My boyfriend and I can't agree on this. We change who gets which side based on the shoulder we'd predominantly sleep on and how it's feeling. This let's us get good cuddles before shoulder pain gets irritated. He comes to bed after me. A while back he asked what side I'm sleeping on. I said "left". Later that night, he comes in and almost lays directly on me because he claims "left" is the other side. Since then we have to describe which side using complicated descriptions.