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Up until a month ago, the dresser drawer beside Rick Theis's bed was the holding pen for his nighttime reads: printed editions of Letters to a Young Contrarian, Moneyball and Lords of Finance.

But he's replaced those tomes with three e-books, all stored on his shiny new toy: an iPad. He much prefers these digital texts to their paper predecessors.

"I find the experience of reading on the iPad easier on my eyes," he says.

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As the ambient light gets darker, the device's default settings make it glow brighter - it serves double-duty as an e-reader and a light source.

"It can be a bright beacon in a darkened room," Mr. Theis, a 35-year-old officer with an Ottawa advocacy group, says.

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But exposure to such intense light at night could cause sleep disruption, doctors warn.

While the effects of the iPad have not yet been studied, a large body of research suggests that exposure to bright, blue-range light can disrupt the body's natural sleep and wake cycles.

As the day winds down and it gets dark outside, the brain receives a signal to produce melatonin, which shuts off the alerting centre in the brain. But artificial light - such as the kind emitted by electronic devices - can delay that signal.

"As we approach bedtime, the responsiveness of those little cells in that part of the brain that receives the light information from the retina becomes more sensitive," James MacFarlane, the Toronto Sleep Institute's director of education explains. "As that drops off, these light sources tend to make more of a difference."

The fact that the iPad uses an LCD display similar to a laptop or TV and projects blue-range light concerns neurologist Alon Avidan, associate director of the Sleep Disorders Clinic at the University of California Los Angeles.

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In the past, he'd recommend patients skip the late-night talk shows and Internet surfing before bed in favour of "relaxation activities like reading a boring book." The growth of e-readers has made him rethink that advice, though.

A dim light used to read a book is fairly harmless, he says, "But if they are in a dark room and they are getting emission of photons in front of their eyes through the LCD screen, that could be considerably more alerting."

"Not only are you using more intense light, but the light is emitted at you towards the eyes, instead of from behind your head at the book."

While it has the potential to disrupt the body's circadian rhythm, the iPad's display technology is one of its greatest selling points. It was what helped Vancouverite Gary Ng, the editor of, choose the device over its e-reader competition - Amazon's Kindle and Barnes and Noble's Nook, which use low-powered E Ink displays that closely mimic a printed page.

"I found [the iPad]easier to read because of the screen. It was very clear and very bright," he says. "You do require a light source with the Kindle. You need to read it like a real book [and]use a night light."

Many of the e-reader applications available for the iPad offer a night mode, which displays white text on a black background and projects less of that blue-range light. But because of eye strain, many users prefer using the regular settings for their late-night reading sessions.

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Duane Storey, a 33-year-old Vancouver web designer, says he reads e-books off his iPad with the product's default settings: black text on a white background. He test-drove the Kindle and Nook, but preferred having his books and light source available in one device.

"I do read without a light on now before bed," he says.

He usually reads an hour on weeknights before bed, and up to eight hours throughout the day on weekends.

"I haven't read before bed in a while … [the iPad]has encouraged me to read more," he says.

Mr. Storey has only been using his iPad for two and a half weeks and hasn't noticed a marked change in his sleep patterns, but he's had disordered sleep when he's used his Macbook Pro late at night.

"I definitely noticed that for sure before bed if I [did]anything on a laptop," he says.

Like many iPad users, Mr. Ng says the temptation to do a quick e-mail check or Google search in the middle of a reading session on the iPad can be difficult to resist. The range of applications and diversions on the device are another cause for concern among sleep specialists.

"We have all these gadgets that allow you to surf the Web and answer e-mails and when you do that in bed, there is a problem," Dr. Avidan says. "It builds anxiety, it heightens the stress … [the brain is]getting stimulated and it's hard to get to sleep."

Mr. Theis limits most of his end-of-day iPad use to reading a combination of books and work-related articles, but still has the device in bed for at least an hour each night.

He lowered the brightness on his iPad to conserve energy, and occasionally uses it with a table lamp, but "it's more about circumstance than need," he explains.

"It's so I don't feel like I'm 13 again with a flashlight."

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