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How the bio clock tells time

Q: I can wake up at a desired time (plus or minus 15 minutes). Can you explain this ability please? --"Wake us up", Huntsville, Texas

A: Last week I said that we don't know the answer to your question. We are fathoming how bio clocks work but we have much to learn before explaining your ability. However, I promised to explain what we do know about the clock's workings, this week. Let's do it.

[Gray's Anatomy, Bartleby.com] The human brain

Our clock has three parts: a clump of brain cells, called the suprachiasmatic nucleus (SCN for short), light receptors in the eye that talk to the SCN, and a gland that makes hormones when the SCN says so. (Hormones are chemical messengers sent through the bloodstream.)

[Gray's Anatomy, Bartleby.com] Close up: bio clock's elements

Let's review some anatomy. The 4-gram (about the weight of a ruby-throated humming bird) hypothalamus houses the SCN. This brain part sits behind the eyes, directly below the thalamus. See figure, colored yellow. The hypothalamus controls every hormone gland in the body and thereby changes vital body functions: blood pressure, temperature, metabolism, and how much adrenaline pumps through the system.

The hypothalamus causes body operation to ebb and flow in a daily rhythm like the tides. It needs a clock: the SCN.

The SCN uses light and other information to set itself. Light receptors in the eye transmit nerve signals to the brain via the optic nerve (pink). Some of the fibers in the optic nerve go directly to the SCN, linking the SCN to the eye. That's how the SCN gets light data to tell the time of day and the seasons.

The pea-size d pineal body (blue) sits behind the hypothalamus and contains the pineal gland. This gland produces the hormone, melatonin, when the SCN tells it to. Melatonin's role in controlling body functions isn't completely known but increased levels of melatonin do make people sleepy.

The clock elements work together with the hypothalamus to control a body function. For example, let's consider the sleep/wake cycle. The SCN gets light data from the eye sensors, learns the length of day, and passes the information to the pineal gland, which secretes melatonin. Lack of sunlight triggers the pineal gland to produce more melatonin. The hormone acts on the hypothalamus and other brain structures to lull us into longer slumber to match a longer night. That's why we tend to sleep longer in the winter: less light, more melatonin, more sleep.

Light is only one external stimulus which provide our body with time cues, says Samir Bangalore, researcher at Northwestern University Medical School in Chicago. Light is the most "potent" one; ambient temperature and the rest/active cycle are others. Other senses besides sight may contribute to our clock's function (hearing, touch, and taste) but we haven't studied these much, yet.

(Answered Aug. 9, 2002)

Further Surfing:

WonderQuest: What makes us tick

Washington U School of Medicine: Neuroscience tutorial

Science Week, R.L. Sack: Melatonin and circadian rhythms