Recently, the news media has picked up on a study in Current Biology that, supposedly, explains why people get testy after a sleepless night.
In theory, the study shows that people are more emotional, more reactive, and more primitive after sleeplessness. Quoting the news article at Yahoo!:
A shutdown of the prefrontal lobe - a brain region that normally keeps emotions under control -- is the reason for heightened emotional response in sleep-deprived people...
Well, not exactly. The problem isn't with the facts of the story, but the interpretation.
First, the study took a group of people and sleep-deprived them for 35 hours, once. This isn't exactly the most obvious interpretation of "sleepless night," nor is it generalizable to sleeping two hours less than normal. Additionally, this is a one day deprivation. While on one hand it might sound like recurrent sleep deprivation would actually be worse, that isn't necessarily true: for example, how do you know your body and mind don't simply adapt to it?
Second, it sounds like sleeplessness makes you more emotionally reactive; that the "norm" is a low level of emotional reactivity that is artificially magnified by not sleeping. But an equal, though different, interpretation is that good sleep makes you less emotional; that the normal, default state is very emotional, and sleep suppresses this lability.
It sounds like splitting hairs, but it's not.
This issue similarly appears in an earlier article titled, "Sleepless nights may hinder moral judgments". It seemed to report that sleep deprivation (in this case, 53 hours) resulted in difficulty making moral judgments, which certainly sounds negative, and the obvious association is with the military. But the study didn't actually result in this finding. What it found was that sleep deprivation made it more difficult for a person to view a hypothetical situation as morally acceptable. It was still easy - in fact, easier - to identify an act as morally inappropriate. As with the lability study, this study raises the question of what conditions constitute a default condition, versus what requires effort on our part. In this case, deciding if something is right takes effort and moral judgment. Deciding if something is wrong is more instinctive and reflexive.
Back to the emotional lability study: one final important point is that this is an association study, not a discovery of a causal link. The study showed that a certain region of the brain works differently when you are sleep deprived. We think this region of the brain is related to emotions and lability, because it is active in other studies about emotions. However, we don't know if this region causes the emotions, or the emotions cause activity in this region. In other words, the news article makes it seem like sleep deprivation changes the function of this brain region, and so you become labile. It could be that sleep deprivation makes you labile for some other reason; and these emotions change the activity of that brain region.