I recently came across a 2005 survey, done in Texas, that found people with poor sleep were far more likely to be depressed or anxious than people with better sleep. Huge risk ratios:
People with insomnia . . . were 9.82 and 17.35 times as likely to have clinically significant depression and anxiety [than persons without insomnia.]
Other studies have found similar results. For example, a 1979 survey interviewed the same people twice, one year apart. People who had insomnia both times were 40 times more likely to be newly diagnosed with major depression during the intervening year than those who did not have insomnia at either time.
A simple thing to say about the sleep/mood correlation is that it supports my theory of depression, which says depression is often due to malfunction of two circadian oscillators (one controlled by light, the other by faces). If they are working properly (in sync, with large amplitude) you sleep well and are in a good mood when you are awake. If they are not working properly (e.g., not in sync) then you do not sleep well and are in a bad mood at least part of the time while you are awake. What is called depression (e.g., not wanting to do anything) is actually a good thing in the middle of the night. Not wanting to do anything — being still — is necessary to fall asleep.
A sad and more complicated thing about this correlation is that it is ignored. It is not explained by any theory of depression popular among psychotherapists, such as cognitive-behavioral therapy, not to mention a dozen other explanations of depression (psychoanalytic, etc.) that psychotherapists favor. Nor is it explained by any pharmacological theory of depression. In other words, if you seek treatment for depression within our healthcare system the treatment you will receive will derive from a theory that cannot explain this result. Yet the correlation is so strong it must be telling us something important.
You can read endlessly about the high cost of health care. What if the high cost is not the core problem? What if it is only a symptom of something less obvious? What if health care costs a lot because we have a poor understanding of health and disease (as the failure of popular theories of depression to explain the sleep/mood correlation suggests)? What if we have a poor understanding of health and disease because health research is too concerned with allowing healthcare providers to make money?