Pain and Sleep: Understanding the Interrelationship
Do poor sleep patterns influence the development of future chronic pain? To answer this question, Finan et al selected 17 well-designed studies.2 They included 3 large longitudinal studies ranging from 1 to 12 years that indicate that elevated insomnia symptoms increase the risk of headache.4-6 It is important to note that the research was selective to tension-type headache and not migraine. Another population study of 15,350 Norwegian women found that sleep disorders predicted the development of fibromyalgia 10 years later.7 The authors of this study estimated that two-thirds of the patients in the sample who were diagnosed with fibromyalgia had preexisting sleep problems. Finan concluded that sleep problems increase the risk of chronic pain in pain-free individuals, worsens the long-term prognosis of existing headache and chronic musculoskeletal pain, and influences daily fluctuations in clinical pain. Conversely, they also mention that good sleep appears to improve the long-term prognosis of individuals with chronic pain conditions.2
Next, Finan et al evaluated recent prospective studies assessing the bidirectional effects of sleep and pain. They proposed that a trend has emerged suggesting that sleep disturbances may predict pain to a greater degree than pain predicts sleep problems. Their broad analysis of data suggests that sleep and pain appear to be a reciprocally related, but a closer analysis suggests that poor sleep may have a stronger influence on the experience of chronic pain.2
The growing body of research strongly suggests, at the clinical level, that sleep quality should be included in the initial assessment of all patients who present with chronic pain. In my opinion, this is not a difficult task. For the past 25 years, I have included interview questions that assess the quality of a patient’s sleep. If I determine that a sleep problem exists, I will ask the patient to fill out a sleep log that covers 1 week. Further, I will administer the Epworth Sleepiness Scale (ESS).8 This scale is easy to administer, takes 10 minutes to fill out, and provides additional information to help rule out whether a sleep disorder exists. I include this information in my report back to the referring physician, so they can decide if a formal sleep study is indicated. The ESS can be obtained free of charge online.8
Prevalence of Sleep Problems
One finding consistently stands out based on large community studies from around the world—chronic pain appears to be the main reason why patients sleep poorly (difficulty initiating sleep, disrupted sleep, early morning awakenings, and unrefreshing sleep). Because there has been no uniform methodology in pain/sleep epidemiological studies, the estimated prevalence of sleep problems is quite variable, ranging from 23% in Europe to 89% in the United States.9 Table 1 provides a summary of sleep disturbance findings in patients with chronic pain. Research supports the fact that pain and sleep are interrelated, and the relationship is very complex due to the many factors that can influence both processes.
The Nature of Sleep
Since sleep and pain are reciprocally related, both bidirectionally and unidirectionally, it is important to understand the nature of sleep—the types and patterns of sleep, how sleep is generated by the brain, and the role that sleep plays in an individual’s overall functioning. It is necessary to understand the mechanics and physiology of sleep to be able to appreciate the interrelationship of pain and sleep on physical activity and behavior.
What is sleep and why is it so important to the patient with pain and in the treatment of pain? The average individual spends a considerable amount of time sleeping. About one-third of our life is spent in this endeavor. In the past, it was thought that sleep was quiet time for the brain and body to recuperate from the demands of the day. Recent sleep research, however, has refined that notion and has discovered that sleep is a carefully controlled and highly regulated series of states that occur in a cyclical fashion each night. Sleep is critical in maintaining homeostasis, which reinforces the fact that we need uninterrupted sleep to survive and cope with the demands of every day life. Quality sleep is crucial for the pain patient who deals with the additional demands that pain imposes on maintaining homeostatic balance.
Sleep is divided into 2 separate and distinct states: rapid eye movement (REM) and non-REM sleep. Both are equally important in maintaining physical and mental homeostasis. When a patient undergoes a sleep study, brain wave activity is monitored by an electroencephalogram (EEG). This information helps the sleep physician determine the quality and quantity of both REM and non-REM sleep.
Non-REM sleep is divided into four stages: Stage I and II are considered “light sleep,” whereas Stages III and IV are considered “deep sleep.” As we progress from Stage I to Stage IV, brain wave activity slows down, as measured by the EEG, from active beta-wave to slow delta-wave sleep. Non-REM sleep is important for both brain and body restoration.
REM sleep is considered “active sleep” because brain activity and EEG patterns are similar to brain activity when we are awake. During REM sleep, our muscle systems are inactive, unlike heart rate, breathing, and blood pressure, which are highly variable. Also during REM sleep, we experience story-like dream activity. In my opinion, it is our ability to dream that is critical in maintaining mental homeostasis. Dreaming allows us to deal with the demands of life, and when REM sleep is disrupted, mental disturbances can result. I will revisit this issue later when pain and sleep medicines are reviewed.