If you've personally suffered from or known someone who has, you know that when people are really depressed, they have a strong urge to stay in bed.

Why do depressed people lie in bed? It isn't because of great snuggle time under the blankets. It's because depressed people can't bring themselves to get out of bed. Almost any activity or task becomes a painful ordeal, even things as simple as taking a shower or getting dressed. A perfectly able-bodied person can't even bring him or herself to rise out of bed. How does this happen?

The intuitive answer is that a lack of is to blame. Depressed people are directionless because they are undercommitted to goals. Without goals to drive future behavior, current behavior becomes frozen for long periods. Beds are the most natural location for a behavioral pause, as the place in the house most associated with inactivity.

The intuitive answer is okay as far as it goes. The problem is that it just doesn't go very far. It begs the question of how a person loses the desire to pursue goals in the first place. The answer involves a surprising theory that takes us closer to understanding how it is that low moods intensify into more serious episodes of depression.

First, we have to detour to contemporary , which tells us that moods have a function: Moods help us pursue goals efficiently. High moods help us to more vigorously pursue rewards. Low moods tell us when our progress towards goals is poor. Often, low moods first arise when we've hit an obstacle, or when an important goal is threatened. Our usual first reaction to a low mood is to redouble effort towards the blocked goal. If the goal still proves to be unreachable, the low mood will escalate. At some point, something has to give: Usually the person will give up, or scale back on the goal and/or move on to another activity that has a better pay off. Authors such as Randolph Nesse and Eric Klinger have made a powerful case for the utility of low mood. In a world where time, resources, and effort itself are all precious and finite, having an evolved mechanism to hasten disengagement from a failing goal is very important to survival.

These relationships between moods, goal, and effort hold for a variety of species. A bear fishing for salmon without luck in a favorite river bend uses low mood to help it move on to another spot. For better or worse, human is more complicated because we can choose either to act or not to act on our mood. I believe that humans are the only species that can decide to ignore low mood and to continue pursuit of an unreachable goal. In a sense, this creates the potential for a standoff between the person and their ancient mood system. To resolve the standoff, the mood system must do something more drastic: It turns down the volume on goal pursuit, not only on the one goal but on goal pursuit across the board. Eventually, when the mood system wins the result is flat-on-your-back depression, with fatigue, torpor, a lack of motivation, the whole nine yards.

This alternative theory turns the standard explanation on its head. Depressed people don't end up in bed because they are undercommitted to goals. They end up lying in bed because they are overcommitted to goals that are failing badly. The idea that depressed people cannot disengage efforts from failure is a relatively new theory. It has not been much tested in research studies. However, the idea is well worth exploring. It fits well clinically with the kinds of situations that often precipitate serious depression—the battered wife who cannot bring herself to leave her troubled , the seriously injured who cannot bring himself to retire, the laid-off employee who cannot bring herself to abandon her chosen despite a lack of positions in her line of work. Seeing these depressions in terms of unreachable goals may be useful clinically, and may help us better understand how ordinary low moods can escalate into incapacitating bouts of depression.

Jonathan Rottenberg is the author of The Depths: The Evolutionary Origins of the Depression Epidemic, available where books are sold.