With the start of a new semester comes a return to classes, friends and parties, all of which demand a chunk of time and often cut into time allotted for sleep. According to the 2005 National Sleep Foundation's Sleep in America Poll, 21 percent of those between the ages of 18 and 29 reported getting less sleep than they needed. "A problem with a lot of college students is sleep deprivation," said Dr. Pearl Yu, an assistant professor of pediatrics at the University who is board-certified in sleep medicine. One reason for this may be delayed sleep phase, a chronic disorder of sleep timing which affects most normal teenagers, according to Dr. Robyn Boedefeld, assistant professor of medicine at the University. "It is the tendency to go to bed late and sleep late," Boedefeld said. Those with delayed sleep phase can easily fall asleep at the same time every night but have difficulty waking up at the desired time the next morning. Boedefeld explained that it can cause problems when a student tries to go to work or class. According to Dr. Cynthia Brown, assistant professor of medicine at the University, delayed sleep phase syndrome is the most common sleep disorder among college-aged students. "The circadian scale shifts to where most college-aged individuals tend to be night owls," Brown said. According to the 2005 Sleep in America Poll, 59 percent of those between 18 and 29 consider themselves evening persons. Rachel Nablo, a third-year in the Education School, said this description fits her. "Since coming to college, I can never seem to go to bed before midnight, even if I have something important to do or something early the next day," she said. When students do not get enough sleep at night, it can be tempting to nap during the day. Yu said, however, that college students should not need to nap if they have a "routine." She also pointed out that students who suffer from sleep inertia, a feeling of extreme grogginess after awakening from a deep sleep, should avoid napping, which might result in difficult waking up and getting going again when they need to. Nablo said she believes sleep inertia may be a problem she has after napping. "Sometimes naps do more harm than help, like when you can't seem to recover from them and stay groggy for a couple of hours after," Nablo said. "I usually try not to nap unless I have a big enough window of time to fully wake up again before something important." Yet Yu added that not everyone faces this problem and that naps can be helpful for some. "You want to nap so that you feel refreshed," said Yu. "If naps are restorative for you then by all means do it." The amount of time needed for the ideal nap varies depending on the person. "Some people sleep for 20 minutes and feel horrible, and some feel great, so it's variable from person to person," Yu said. Third-year College student Erin Moran said her ideal nap length is generally less than an hour. "Anything more than that, and I wake up more tired than when I went to sleep," Moran said. Another sleep problem is obstructive sleep apnea, which is characterized by difficulty breathing at night and snoring, and is also associated with obesity. According to Boedefeld, OSA afflicts four percent of men and two percent of women and can be caused by genetic factors as well as facial characteristics. Brown explained that OSA occurs when the tongue falls back so that it obstructs the throat. There is treatment available for OSA, but diagnosis can only be made after a doctor has observed the sleep, according to Brown. "The most common therapy is a nasal [continuous positive airway pressure machine]," Brown said. "The mask sits over the nose while you are asleep and blows air at a certain pressure so that the throat can no longer collapse while you are sleeping." Though many who have OSA are not aware of their symptoms, it is important to get treatment. "Untreated cases of OSA can increase your likelihood of having high blood pressure or stroke," Brown said. Chronic sleep loss can also be detrimental to one's health, possibly causing depression or memory problems, Boedefeld said. According to Yu, severe sleep deprivation can lead some to exhibit psychotic behaviors, such as hallucinations?. According to Yu, there is no way to identify a cure-all rule for how much sleep a person must get in order to function and avoid such behaviors. "It varies from person to person how much sleep deprivation a person can tolerate," Yu said. "It depends on the person's needed amount of sleep and how much deprivation they can tolerate." A study published in the Journal of the American Medical Association in 2005 found that those who restricted their sleep to four hours a night had the same performance on a simulated driving task as those who slept six hours a night and were given alcohol to bring their blood alcohol content to .05 percent. This indicates that fatigue may impair driving ability in the same way that consuming alcoholic beverages does. According to Boedefeld, many people try to get by on only five to six hours of sleep each night, while most people need around eight. Yu emphasized the importance of good "sleep hygiene," which she defined as "getting to bed at a routine time every night, not drinking caffeine late in the day and not exercising before bed." Additionally, it is best to sleep in the dark because darkness signals the brain that it is time for bed. Conversely, bright lights, such as the glow of a computer screen, can stimulate hormones that promote wakefulness, according to Brown. Brown added that to help wind down, many people have an alcoholic drink before bed, thinking it will help them to fall asleep more quickly. She noted that while this method may work, the practice does not promote healthy sleep. "It suppresses the ability to have rapid eye movement, or REM, sleep," Brown said. "It won't help you stay asleep; in fact, it will break up your sleep a little bit more." For those looking to fall asleep faster, it is helpful for the internal body temperature to become cooler. "Warm milk can dilate blood vessels in the body to release heat, lowering internal body temperature," Brown said. She added that exercise right before bedtime is not a good idea since it raises body temperature. The new semester might mean cutting back on much-needed sleep, but Yu advised students to avoid becoming sleep-deprived. And while weekends might provide a tempting opportunity to catch up on sleep, she pointed out that there is no way to fully regain lost sleep.