Historically, insomnia has been thought of as secondary to other disorders such as depression. The idea was that you became depressed – and that your sleep got messed up as a consequence. This might involve difficulty falling asleep, excessive time awake at night or waking up earlier than hoped.
This may make sense to those who have experienced depression and found that thoughts of distressing events such as of a deceased loved one, or previous failures, keep them awake at night. The possibility that depression leads to insomnia is also consistent with research in which I have been involved – where we found that adults with insomnia were more likely than others to have experienced anxiety and depression earlier in life.
But could things really be the other way around? Could poor sleep be making you depressed? Over the past decade or so it has become increasingly clear that disturbed sleep often comes before an episode of depression, not afterwards, helping to do away with the notion that sleep problems are secondary to other disorders.
This is not too hard to relate to either – just think about how you feel after you have slept poorly. Perhaps you feel tearful or snap at those around you. The literature seems to back up the idea that our ability to regulate our emotions is reduced after a bad night’s sleep. Insomnia has also been shown to predict depression defined according to diagnostic criteria.
So why does poor sleep lead to depression? Lots of different mechanisms have been proposed. To give just a few examples, let’s start by thinking about our behaviour. I, for one, am more likely to cancel an evening out with friends or an exercise class after a poor night’s sleep. This could be part of the problem, as such events are exactly those that may help to keep depressive symptoms at bay.
If we think about what happens to the brain when we miss sleep, there are clues as to why sleep and depression are linked. One study on this topic focused on an area of the brain called the amygdala. This is an almond-shaped structure located deep in the brain that is believed to play an important role in our emotions and anxiety levels.
It was found that participants who had been sleep deprived for approximately 35 hours showed a greater amygdala response when presented with emotionally negative pictures when compared to those who had not been sleep deprived. Interestingly, links with parts of the brain that regulate the amygdala seemed weaker, too – meaning that the participants were perhaps less able to control their emotions. Such findings could help to explain how poor sleep may actually cause difficulties such as depression.