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Sleep Disorders II: Disorders of Sleep Rhythm
Sleep is a state of decreased arousal and responsiveness to environmental stimuli.  Sleep and wakefulness occur in cycles under the direction of an internal clock in the brain, the suprachiasmatic nucleus, or SCN.  Although almost all cells in the body have their own internal clock, the SCN sends signals that control all these clocks, hence the SCN is often referred to as the master clock.  This allows sleep and wakefulness to be states that occur throughout the entire body at any given time. 

The SCN works on a cycle that is slightly longer than the 24 hour day of our experience.  However, it is under the influence of the 24 hour day/night cycle through environmental cues called zeitgebers, or “time givers,” the most influential of which is light.   This dynamic tension between the inherent tendency of the SCN to expand its period against the day/night cycle that constrains it lends remarkable stability to our sleep-wake cycle. 

Sometimes, however, our sleep/wake cycle can get out of sync, or phase, with the day/night cycle.  The reasons for this are many.  Some blind people lack the ability to perceive light; their SCN cannot tell the difference between night and day.   Consequently, such people have a “non-24-hour” sleep/wake rhythm that periodically puts them at odds with the day/night cycle.  Symptoms include daytime sleepiness, trouble sleeping at night, and difficulty with memory and concentration.  There are medications that can help such people synchronize with, or entrain, the day/night cycle.   

Shiftworkers are required to be awake when the majority of people are asleep, and vice-versa.  However, shift work itself rarely resets the SCN, so these workers are awake when their body wants to sleep, and must sleep when the body wants to be awake.  Further complicating matters for shift workers is that social demands frequently require them to be awake during their sleep period, reinforcing the SCN remaining entrained to the day/night cycle.  The net effect is that shift work often leaves workers chronically sleep deprived and less than optimally alert during their shift.  Medication is available to help them maintain alertness during their shift, and allow them to remain awake so they can bring more of the urge to sleep (sleep pressure) to their sleep period. 

Some people are troubled by being unable to fall asleep early in the night, preferring to stay awake into the early hours of the morning, when they can then sleep with ease – although awakening before later in the morning can be quite difficult.  Such people are “night owls,” more technically suffering from delayed sleep phase syndrome.  Anyone with teenagers can testify that going through a period of delayed sleep phase syndrome is a normative part of development.  Opposite the “night owls” are the “morning larks,” referring to those who arise earlier than desired and likewise sleep earlier in the evening.  “Morning larks” are more technically suffering from advanced sleep phase syndrome.    This syndrome is especially prevalent amongst the elderly. 

Disorders of sleep phase can be treated with a combination of behavioral changes, or sleep hygiene (see below) and medication.  The medicationmelatonin is a hormone the body produces in response to darkness.  At certain points in the SCN’s cycle, the SCN is particularly sensitive to the effects of melatonin, which serves to advance or retard the SCN’s rhythm. In particular, the body’s core temperature is a very sensitive indicator of the state of the SCN’s cycle.  Late in the night, when the body temperature is at its lowest, or nadir, the SCN is at its most quiescent.  Melatonin administered prior to this core body temperature nadir tends to advance the sleep phase, whereas melatonin given after the nadir tends to retard the sleep phase.  This logic helps explain how melatonin is used to treat disorders of sleep phase:  In delayed sleep phase disorder, the goal of treatment is to advance the sleep phase, to which end the melatonin is taken late in the afternoon; in advanced sleep phase disorder, the goal is to retard the sleep phase, thus melatonin is taken upon arising each morning. The effects of melatonin, while powerful, are not as powerful as light; and since light affects the SCN in a manner opposite of melatonin, it is important to avoid bright light in the time following the administration of the melatonin to optimize its effectiveness.   

Finally, modern travel makes it possible for the SCN to be exposed to radical changes in the local day/night cycle that it does not have time to adjust to. Westward flight, or jet lead, has the effect of causing advanced sleep phase syndrome – that is, wanting to go to sleep and awaken earlier than the local time.  The use of bright light and activity to remain awake during the daylight period of the local time is often sufficient to make the adjustment, at a rate of one day per westward time zone crossed, because of the SCN’s inherent pressure to maintain wakefulness longer than necessary for a 24-hour day/night cycle.  That is, staying awake later is easier than trying to go to sleep earlier because staying awake is essentially going along with the SCNs inherent tendency to run long relative to the local daylight period. Eastward flight causes jet lag, the net effect of which is to cause delayed sleep phase syndrome relative to the local time.  This is the more difficult situation to deal with as it easier to move the SCN forward rather than back, due to its propensity to expand against the constraining effects of the day/night cycle.  It can best be managed by using mild sleep aids, darkness, and melatonin late in the daylight portion of the local day/night cycle to induce sleep at the appropriate local time; entrainment of the local day/night cycle by the SCN should follow at the rate of one day per eastward time zone crossed. 

Sleep hygiene involves a regimen of behaviors and environmental adjustments conducive to sleep at the appropriate point in the local day/night cycle.  Sleep hygiene includes avoiding bright light late in the day; avoiding large evening meals; avoiding sleep disrupters such as caffeine, nicotine, alcohol, and too much liquid near bedtime; assuring that the bedroom is dark, quiet, and used only for sleep; establishing a bedtime routine that involves reducing mental stimulation such as light reading, or making a list of worries and concerns then putting it aside; taking a bath or otherwise engaging in relaxing behavior – in general, avoiding physical or mental stressors or activation; waiting to get into bed until sleep is the only option – and getting out of bed if unable to fall asleep; and turning the clock around and turning off the chimes or otherwise eliminating indicators of the time of night.

Author: Mark J. Woyshville MD FAASM

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