I promised to blog more about research, and I will. Unfortunately, in the one week between my world tour and the start of the fall semester, I’ve been spending less time on quantum complexity research than on sleeping on a new mattress that I bought. This has provided ample time to ponder the following question, which I’ve decided to add to the Shtetl-Optimized Physics for Doofuses series:
Why is a soft bed more comfortable than a hard one?
At first glance, this question seems too doofusy even for a series such as this, which makes its target audience clear. The trouble is that, while perfectly reasonable-sounding answers immediately suggest themselves, several of those answers can be shown to be wrong.
Let’s start with the most common answer: a soft bed is more comfortable than a hard bed because it molds to your shape. The inadequacy of this answer can be seen by the following thought experiment: lie on a soft bed, and let it mold to your body. Then imagine that the bed retains exactly the same molded shape, but is replaced by ceramic. No longer so comfortable!
Ah, you reply, but that’s because a ceramic bed doesn’t change its shape as you shift positions throughout the night. But this reply is still inadequate—since even if you’re lying as still as possible, it still seems clear that a soft bed is more comfortable than a hard one.
So it seems any answer needs to start from the observation that, even when you’re lying still, you’re not really lying still: you’re breathing in and out, there are tiny vibrations, etc. The real point of a soft bed is to create a gentler potential well, which absorbs the shocks that would otherwise be caused by those sorts of small movements.
(I was tempted to say the point is to damp the movements, but that can’t be right: trampolines are designed for minimal damping, yet sleeping on a trampoline could actually be pretty comfortable. So the essential thing a bed needs to do is simply to make way in response to small movements and vibrations. How hard the bed tries to spring back to its original shape is a secondary question—the answer to which presumably influences, for example, whether you prefer an innerspring or a memory-foam mattress.)
So then why aren’t beds even softer than they are? Well, the limit of infinite softness would be a bed that immediately collapsed to nothing when you lay on it, dropping you to the floor. But even before that limit, a bed that was too soft would give you too much freedom to shift into awkward positions and thereby cause yourself back problems. This suggests an answer to a question raised by a colleague: is the purpose of a bed to approximate, as well as possible on the earth’s surface, the experience of sleeping in zero gravity? Unless I’m mistaken, the answer is no. Sleeping in space would be like sleeping on a bed that was too soft, with the same potential for back problems and so forth.
Given that lying in bed is normally the least active thing we do, I find it ironic that the only reasons we lie in bed in the first place (as opposed to, say, on steel beams) are dynamical: they involve the way the bed responds to continual vibrations and movements.
I’ll be grateful if knowledgeable physicists, physiologists, or sleepers can correct any errors in the above account. Meantime, the next time your spouse, partner, roommate, parent, etc. accuses you of lounging in bed all afternoon like a comatose dog, you can reply that nothing could be further from the truth: rather, inspired by a post on Shtetl-Optimized, you’re struggling to reconcile your modern understanding of the physics and biology of lying in bed with the prescientific, phenomenal experience of lying in bed, and thereby make yourself into a more enlightened human being.