Humans are rhythmic. Every bodily function known cycles predictably across the 24-hour day. The most basic of our behavioral cycles is the sleep-wake cycle. Sleep serves as an "anchor" in the 24-hour day around which our other bodily rhythms -- hormonal, physiological, neurological, and behavioral -- are synchronized. Our internal biological clock (located in the hypothalamus of the brain) runs slightly longer than 24 hours.

What synchronizes it to a 24-hour day is the natural light-dark cycle generated by the rotation of the earth around the sun.Our biological clock relies on light input to literally tell it the correct time of day. Light information coming in through the retina is relayed both back to the visual cortex and forward to the hypothalamus and the pineal gland. The pineal gland -- which releases the hormone Melatonin -- functions as a transducer to convert light into information about the time of day.

This time information is then relayed forward and processed by the clock (the SupraChiasmatic Nucleus) in the hypothalamus.When too little natural bright light gets to the brain or when bright light at the wrong time of day enters the brain, it disrupts the timing of the "clock" which may then incorrectly or fail altogether to synchronize our body's rhythms to a 24-hour day.

The first indication that something has gone wrong with the "clock's" ability to synchronize our rhythms to a 24-hour day is the inability to maintain our sleep period at a normal clock hour, such as 11pm or midnight. This may happen in conditions of shift work and mood disorders such as major depression, manic-depressive disorder, or Seasonal Affective Disorder/winter depression.

Alternatively, it may occur in the elderly where regular day activity is severely restricted, or it may occur in the blind.Except in the blind, we can use light at specific times during the day to "reset" the brain's "clock" to a 24-hour day or to an alternative work shift schedule.

We can also use light to "reset" the timing of the sleep period and decrease sleep state disruptions. In the blind, oral Melatonin can be used in place of light input to "reset" the brain's "clock" to a 24-hour day.

TOP