Studies show that insomnia varies widely in the population, from 8-40%, depending on how the researcher defines insomnia. Dr. Fernandez-Mendoza has differentiated poor sleep from an insomnia disorder in past studies. In a recent study published in the May edition of the Journal Sleep, Dr. Fernandez-Mendoza and others examined the risk factors of poor sleep in a large sample of a Pennsylvania population using both subjective and objective information.
The study participants had a complete sleep history and physical examination along with a sleep study at baseline, and were then followed for an average of 7.5 years. The researchers defined sleep quality as normal, poor sleep (meaning one or more insomnia symptoms, but not enough to qualify for an insomnia disorder), or chronic insomnia. Patients with chronic insomnia at baseline were excluded from the study, as the researchers were studying poor sleep risk factors, not chronic insomnia risk factors.
The results showed that at any time during the study, 18% of participants were poor sleepers. Risk factors for this population included having ulcers (presumably stomach ulcers, although not explicitly defined in the paper), depression, obesity, moderate to severe sleep apnea, and smoking. Interestingly, alcohol consumption did not pose an increased risk. Of note, obesity was an independent risk factor for poor sleep, even in those without sleep apnea. The researchers suggest that metabolic abnormalities from obesity could play a role in development of poor sleep.
Thirty-nine percent were persistently poor sleepers. Risk factors for this population was having a heart "disorder" (type of disorder not specified in this paper), having an ulcer, being middle age, and having allergy/asthma.
Seventeen percent went from being poor sleepers to having chronic insomnia. Risk factors for this population were being female, age less than 40, depression, smoking, fewer physical health conditions, shorter objective sleep duration (as measured by the sleep study), and a family history of sleep problems. The authors point out that the short sleep duration finding could mean that elevated nervous system arousal could be responsible for this and may increase risk of medical problems with this type of insomnia, in contrast to chronic insomnia with a normal objective sleep duration. I wonder if these patients have less of a physiologic insomnia, and instead they could be naturally "short" sleepers (perhaps needing 6 hours of sleep or less per night). They would then develop insomnia by trying to make themselves sleep 8 hours each night.
Finally, 44% went from poor sleep to normal sleep during the study, demonstrating that poor sleep can resolve. It's not clear why, but could be due to improvement in underlying physical condition(s) that were driving the poor sleep to begin with.