You finally have a legitimate reason not to respond to a late-night email from your boss: According to a new study conducted by Michigan State University, people who use their phones for work-related purposes after 9 p.m. don't sleep that well and are less productive the following day.

"We studied work-related phone calls, emails, and text messages because engaging in work before bed doesn't allow people to disengage from their day, unlike more relaxing activities like texting with a friend," lead study author Russell Johnson, an assistant professor of management, tells Yahoo Shine. "We chose to study 9 p.m. because studies show that most people fall asleep anywhere between 9 p.m. and 1 a.m."

Interestingly, when compared to people who engaged in other stimulating activities, such as watching "excessive" amounts of television before bed (two hours of viewing or more), those who used their phones slept the worst. "Smartphones are the most disruptive gadgets because they're portable, so people can place them close by, under the pillow or on the nightstand. And unlike laptops, smartphones beep and light up so they're more distracting and tempting," says Johnson. Another reason not to do a final check of your email before bed: Smartphones emit a blue light that Johnson says is the most stimulating of all colors because it inhibits the production of melatonin, a sleep-inducing chemical.

It's not just your phone that's the problem. Check out a few other ways you might be sabotaging your sleep.

You smoked a cigarette: Bad news for people who light up. Smoking before bedtime disrupts sleep patterns, according to a study published in Chest, the journal of the American College of Chest Physicians. That's because smokers spend more time in "light sleep" mode, not REM sleep, since nicotine triggers the release of stimulating chemicals such as dopamine and serotonin. Study subjects who smoked before bed also reported feeling less rested the next day. If you can quit, great; if not, at least light up a few hours before bed.

Your bra is too tight: Some women prefer the support of a bra in bed, but wearing a tight bra that cuts into the rib cage or shoulders can mess up your sleep, according to a Japanese study. Researchers found that tight garments interfere with circadian rhythms (in simple terms, the sleep cycle), resulting in increased body temperature and decreased melatonin levels, two of sleep's worst enemies.

You ate grilled chicken for dinner: While a protein-rich meal or snack is good for your health, it's actually bad for your sleep, because digesting protein is labor-intensive and often too difficult for the body to pull off during sleep mode. For a peaceful sleep, opt for a starchy carb-rich snack such as cereal or crackers before bed instead. One small Australian study found that carbs spike levels of blood sugar, causing people to fall asleep faster, especially if they've eaten within four hours of bedtime.