Circadian Rhythm Sleep-Wake Disorders

Overview

Everyone has an intrinsic and preferred daily rhythm determined by both genetics and habits. The circadian rhythm influences when we feel sleepy and alert; it also influences how cortisol, digestion, and other hormonal pathways function. The circadian "central clock” is located deep in the brain, in the suprachiasmatic nucleus, which is a small collection of pacemaker nerve cells just behind the eyes. These pacemaker nerve cells are activated by light and express an automatic pattern of activity over the course of 24 hours, driving our wakefulness and sleepiness, along with our other internal rhythms. When darkness falls, our nerve cells signal our circadian rhythm to turn off.

Our central clock is influenced by both our environment and our behaviors. Minor adjustments to our clock help us to stay in sync with our environment and daily schedule; major shifts can cause disruption to our patterns of rest-wake (such as shift work or jet lag). Sometimes our behaviors adversely impact our circadian rhythms and make it more difficult for us to fall asleep and wake up when we want to.

Some examples of circadian rhythm sleep-wake disorders include:

• Advanced sleep-wake phase disorder: The circadian rhythm signals sleep earlier than desired. Individuals often fall asleep easily in the evenings and wake up earlier than they intend to.

• Delayed sleep-wake phase disorder: The circadian rhythm signals sleep later than desired. Individuals often have difficulty falling asleep before midnight and have difficulty waking in the morning.

• Non-24-hour sleep-wake rhythm disorder: These individuals are unable to align their circadian rhythm to the 24-hour light-dark cycle which delays their onset of sleep by 1-2 hour every night. Eventually, their sleep cycle will circle around the clock if they sleep without constraints.

• Shift work sleep disorder: Sleep-wake timings are skewed across the week due to work shifts that take place counter to the light-dark cycle. Exposure to light during the work shift adversely impacts the circadian rhythm and shifts the internal clock, making it difficult to fall or stay asleep on non-work nights.

---

Symptoms

Individuals with circadian rhythm disorders typically present with symptoms of insomnia and daytime sleepiness, depending on their circadian phase. In advanced sleep-wake phase disorder, individuals typically start feeling tired early in the evening and fall asleep with ease but wake early in the morning and have difficulty returning to sleep. In delayed sleep-wake phase disorder, individuals have difficulty falling asleep in the evenings but sleep well through the night and have difficulty waking in the morning.

---

Diagnosis

Diagnosis of circadian rhythm disorders usually relies on clinical history and sleep logs. Patterns of activity can be observed using actigraphy or wearable devices that record activity patterns. Melatonin measurements are also useful.