What does a normal circadian rhythm look like?
The typical adult circadian rhythm follows a cycle that is slightly longer than 24 hours - 24 and 11 minutes, on average - and is partly "entrained," or synchronized, to the rising and setting of the sun. Photoreceptors in our retinas send light-dependent signals to our brain's central clock, the suprachiasmatic nucleus, which regulates body temperature rhythms, hormone release, and important physiological functions, including the wake/sleep cycle.

The circadian rhythm for a typical morning riser passes through the following general stages:

6:45 - 7:30 a.m. Around dawn, blood pressure rises sharply, hormones are released, and melatonin secretion is suppressed, causing wakefulness.
10:00 a.m. The body's metabolism speeds up, with alertness increasing through mid-morning.
12:00 p.m. Appetite increases.
2:00 - 5:00 p.m. Physical and mental states (muscular strength, cardiovascular efficiency, reaction times) peak through mid-to-late afternoon.
6:30 - 7:00 p.m. Towards sunset, blood pressure and body temperature reach their highest points during the daily cycle.
9:00 p.m. After darkness, metabolism slows and melatonin secretion begins, preparing the body for sleep.
10:00 p.m. Sleep begins.
2:00 a.m. Deepest sleep.
4:30 a.m. Lowest body temperature.

Does this normal circadian rhythm apply to everyone?
While our internal clocks are universally influenced by the daily cycle of light and dark, we all have our own variation on the standard rhythm. Chronobiologists and circadian researchers have recognized the existence of "night owls" and "morning larks." Some people are genetically predisposed to stay up later, others, to rise early.

How should I plan my lighting at home and in the office?
If you are attempting to minimize circadian disruption, the one rule to remember is that blue light at night is bad. The same wavelengths we see in a bright blue morning sky, which wakes our body up to the day, can cause problems after dark. While it's true that exposure to any light at night may disrupt our biological clocks, light in the blue spectrum, especially between roughly 450 and 480 nm, has been shown to be uniquely effective at suppressing the secretion of melatonin - the hormone which makes us feel sleepy and regulates a number of biological rhythms and functions essential to our health.

Classic incandescent bulbs, which emit light when an electric current passes through a filament, produce light that is "warmer," and more intensely yellow and red. But these are being phased out in favor of energy-saving alternatives, like Compact Fluorescent (CFL) bulbs and Light Emitting Diodes (LEDs), which tend to produce more intense levels of melatonin-suppressing blue wavelengths. Fluorescent lighting often found in offices also has concentrated blue light.

So how should you plan your lighting strategy?


• The first thing involves no bulbs at all: Get at least 15 minutes of natural sunlight every morning.
• At work, make sure to use bright, full-spectrum white light bulbs. Many offices are too poorly lit to help reset the body's daytime clock and to suppress melatonin. Nothing, however, is better than a big window and natural light.


• Avoid using CFL and LED lights in your bedroom and bathroom at night, and avoid extensive exposure to blue light-intensive bulbs and electronics, such as computers and LED TVs, before getting into bed. It's a good idea to turn all of these things off well inadvance.
• If you read in bed, consider reading by dim incandescent light, which produces frequencies shifting towards the yellow and red side of the spectrum.
• Sleep in complete darkness and cover all sources of light (windows, alarm clocks, computers, phones, TVs, etc).
• Remember that if the lights are out, but you can still see your hand in front of your face, there may still be enough light to disrupt your system.

What common sources of lighting are thought to be most disruptive?
Fluorescent / compact fluorescent and LED lighting, which produce more intense frequencies in the blue side of the light spectrum, have been found to be especially potent at suppressing melatonin secretion. LEDs - and devices that use them in their screens, like many TVs and computers - have been singled out in particular. Researchers have found that viewing these electronic devices for only 2 hours may delay melatonin production for an additional 90 minutes after the lights are out.

Will turning down the brightness on my computer screen help minimize circadian disruption?
Dimming electronic devices that have been linked to melatonin suppression has been recommended by researchers studying the effects of computer and tablet-produced light-at-night. The less intense the light, the less likely it is to disrupt your internal clock. There are apps available that will automatically dim your tablet or computer screen and change the color temperature to a warmer hue. While these have not yet been clinically tested, they may be effective in preventing melatonin disruption.

What else can I do to protect my natural circadian rhythms?
While research is underway to create low-energy nighttime lighting that will not disrupt circadian cycles, there are a number of simple strategies to keep your rhythms on track:

• Get healthy amounts of daylight
• Maintain a steady 24 hour rhythm
• If you work night shifts or are exposed to blue-intensive light at night, consider trying light-blocking glasses that have been developed to filter out harmful wavelengths
• Turn out the lights!


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