DULUTH, Minn.-Shadab Rahman's business is sleep, but it wasn't his dream job.

"I needed a summer research project," the Harvard Medical School instructor said. "The only available lab was in Toronto. ... They studied sleep."

That was when Rahman, now 36, was an undergraduate with an interest in cardiovascular medicine. His summer in Toronto led to a second summer as a research associate at the same lab and then work at another Toronto lab with the same mentor as he achieved his doctorate degree.

Now an expert on the effect of light on sleep and circadian rhythm - the "body clock" that guides our daily patterns - Rahman will be in Duluth next month as one of the speakers during a Celebrate the Night Sky symposium.

Like organizers of that week of activities, Rahman is an advocate of darkness at night. Excess light at the wrong times of day, Rahman said recently during a phone interview from his office in Boston, "is a major problem."

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During the interview, Rahman talked about light and darkness, sleep and not being able to sleep, our circadian rhythm and how it gets disrupted.

Here are some samplers:

Blue light's special

It turns out that blue light, not red or orange or green, is the light of choice for your intrinsically photosensitive retinal ganglion cells, also known as IPRGCs.

If you didn't know you had IPRGCs, don't feel bad. They weren't discovered until the early 2000s, Rahman said. They're lodged in the retina, and they have special photoreceptors called melanopsin. That's what triggers our body clock's response to light, Rahman said, and melanopsin's strongest response is to blue light.

Practical application: Rahman conducted a study in Toronto involving nurses who do shift work. The study involved using "blue blockers" to filter out blue light in an attempt to neutralize the disruption in their circadian rhythm. "Obviously every study has caveats, but we got some favorable results where the nurses were sleeping better after they used the intervention," he said.

The flip side is that blue light can enhance alertness, Rahman said. It's as effective as caffeine and preferable if you don't want to stay awake later - because the effect of blue light wears off much more quickly than the effect of caffeine.

7½ hours

There's no getting around the National Sleep Foundation recommendation for seven to eight hours of sleep, Rahman said. "Really, they recommend 7½, but that's for adults," he said. "For adolescents, it's much longer" - between nine and 10 hours.

But what about those highly successful people who say they get by on six, five or four hours of sleep at night?

It's artificial wakefulness, according to Rahman.

"The short-term effects (of being sleep-deprived) are typically increased sleepiness, drowsiness, cognitive detriments," he said. "And what's our solution to that? Drink coffee. ... In the United States, we are the highest consumers of coffee. We chug down coffee all day. People are drinking coffee morning, afternoon, evening, just to stay awake. That just goes to show how sleep-deprived we are."

Does he practice what he preaches?

Yes, Rahman said. He tries to get 7½ hours of sleep at night. He drinks coffee, but only in the morning. "But having said that, I also have a 6-month-old and a 2½-year-old at home, so that's not ideal conditions for sleep."

The 24.2-hour clock

We come with a body clock that runs on a 24-hour cycle - nearly, Rahman said.

Your personal body clock might run on a 23.5-hour cycle or a 24.5-hour cycle. The average body clock cycle is 24.2 hours.

Light allows your body clock to adjust to make up the difference, Rahman said. But it's a problem for some people who are blind - those whose retinas have been destroyed. If that's you, and you have a 24.2-hour body clock, you'll be off by 12 minutes on any given day. In 10 days, you'll be off by 120 minutes - two hours.

It's called "clock drift."

"So if you go to sleep at midnight tonight and your body clock can't adjust to the light-dark cycle ... then in 10 days you're going to ... feel sleepy at 2 a.m.," he said.

Incidentally, the 24-hour clock doesn't apply just to humans, or just to mammals, but to all living beings, Rahman said.

"There are clocks in tube worms that are miles under the ocean," he said. "They are never exposed to light, and yet they have a body clock."

Sleep in space

Astronauts in the International Space Station have two challenges to their circadian rhythm, Rahman said. They do shift work. And because they're circling the Earth, they have a 90-minute light-dark cycle. "And the human body is such that the body clock cannot adapt to a 90-minute light-dark cycle."

With NASA funding, Rahman and other researchers tested new LED lights for the space station to see what effect they might have on circadian rhythm. That study was done on the ground, but a second phase is underway in which use of the lights is being studied in the space station itself.

Too much/too little

Many of us not only get too much light at night, we don't get enough during the day, Rahman said.

"We need light during the day," he said. "And that's why I always say you can't get enough light during the day if you're indoors."

That's because our ancestors spent their days outside, getting up to a million lux of light from the sun. Even a very bright light intensity indoors is only about 2,000 lux, Rahman said. If your office is in a basement and you have no windows, you might be getting 200-300 lux.

Conversely, the brightest campfires our ancestors lit after the sun went down produced no more than 5 lux of light, Rahman said. That much has a negligible effect on the body clock, he said. "But now that artificial light has come in you're getting exposed to 100 lux, 200 lux indoors at night."

All of that artificial light isn't just a bad thing for people who want to see the stars at night, according to Rahman. It disrupts our body clocks, and that negatively affects our health.

"Whenever we study clock disruption, it is obvious that it's associated with various different health disorders ranging from depression to diabetes, from various food disorders to cancer," he said. "You name it, clock disruption is bad."

First-night blues

Do you have trouble sleeping on your first night in a new place? There's a reason for that.

"First-night effect" invariably shows up in sleep studies, Rahman said. Their worst night of sleep will be their first night in the lab.

Our brains evolved that way, Rahman said. A new environment throws off the brain because it's always monitoring its surroundings for signs of danger, even when you're in the sleep state. It's not a deep sleep, and the brain continues evaluating unfamiliar sounds.

"Hotels can spend millions trying to give you the soft pillow, the hard pillow, this bed, that bed," Rahman said. "But it's not your environment. It's difficult to adjust overnight."

Some travelers compensate by bringing their own pillow, he said. Someone used to an urban environment might benefit from a "white noise" app if spending a night in a quiet, rural place.

"Light has profound effects on our biology other than vision," Rahman said. "And it can have a stimulating effect, and we should use it wisely. Just because we have artificial light doesn't mean that we can just abuse it."