A very common yet overlooked cause is putting too much effort into trying to get to sleep:
Via Dreamland: Adventures in the Strange Science of Sleep:
The cause is often the brain’s refusal to give up its unequaled ability to think about itself, a meta-phenomenon that Harvard professor Daniel M. Wegner has called “the ironic process of mental control.” To illustrate this concept, imagine someone telling you that you will be judged on how quickly you can relax. Your initial reaction most likely is to tighten up. After he posed that challenge to research subjects, Wegner found that the average person becomes anxious as his or her mind constantly monitors its progress toward its goal, caught up in the second-by-second process of self-assessment. In the same way, sleep becomes more elusive as a person’s sleep needs become more urgent. This problem compounds itself each night, leading to a state of chronic insomnia.
And Wegner demonstrated the theory with a study:
As predicted, subjects who had been told to fall asleep quickly took longer to do so. Their minds were so focused on falling asleep in record time that they found themselves consciously checking on their progress, unable to let their thoughts drift off and guide them to dreamland.
What’s makes the irony that much deeper is that the people who are prone to this are those who value sleep the most.
They want it so bad that they struggle to get it, and the struggle stops them from achieving their goal:
Patients with insomnia tend to think that one night of poor sleep leads to immediate health problems or has an outsized impact on their mood the next day, a mental pressure cooker that leaves them fretting that every second they are awake in the middle of the night is another grain of salt in the wound. In the inverted logic of the condition, sleep is extremely important to someone with insomnia. Therefore, the person with insomnia can’t get sleep.
Very often people give advice like “when you stop trying so hard things work themselves out.” I usually regard that perspective with great skepticism, but with insomnia it is very likely the case.
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