(CNN)Is falling asleep when your head hits the pillow what you're dreaming of? A fast fall into the land of nod may seem like nirvana, but it's not a sign of a healthy sleeper.
"The well-rested person does not fall asleep immediately," said sleep specialist Rebecca Robbins, an instructor in the division of sleep medicine for Harvard Medical School.
"Falling asleep does take about 15 minutes for the healthy sleeper," Robbins added. "Falling asleep is distinct from sleep itself, which can be frustrating when one is extremely tired.
"Nevertheless, be patient that sleep will come and the more you stress about not being asleep, the lower your chances of falling asleep," she said.
Don't conk out, drift away
Dozing off too quickly could be a sign that you are seriously sleep deprived, which can harm your physical and mental health.
Robbins, who coauthored the book "Sleep for Success!," equates it to having been deprived of food. "If you are starved for food, you will devour your next meal immediately, whereas a well-nourished person might not be as ravenous and in dire need of nutrition immediately," she said.
Adults need to sleep at least seven hours a night, while school-age kids need nine to 12 hours and teens need eight to 10 hours each night, according to the US Centers on Disease Control and Prevention.
What if you do sleep an adequate number of hours each night? Then falling asleep too quickly, as well as being tired during the day, could be a sign that the quality of your sleep is suffering.
"A lack of quality sleep occurs when there are multiple awakenings and arousal during the night," said Dr. Raj Dasgupta, an assistant professor of clinical medicine at the Keck School of Medicine at the University of Southern California, who specializes in sleep.
"Those awakenings affect your ability to get to the deeper stages of sleep, such as slow wave sleep, also known as delta sleep, or rapid eye movement (REM) sleep, which are both essential if you want to function well and be alert," Dasgupta said.
During the REM stage of sleep, we dream -- and information and experiences are consolidated and stored in memory. In addition to impacting cognitive functioning, a recent study found spending less time in REM sleep is linked to a greater overall risk of death from any cause.
Slow wave or delta sleep is when the brain both rests and gets rid of toxins -- a form of housekeeping that allows the body to heal and rejuvenate.
"The most important thing that you can do to increase your amount of deep sleep is to allow yourself adequate total sleep time," according to the American Sleep Association.
One of the most common culprits that might interrupt your sleep during the night -- sometimes without you even knowing it -- is obstructive sleep apnea, where you snore, choke, gasp or stop breathing periodically during the night. Some 25 million Americans suffer from this form of sleep apnea, said the National Healthy Sleep Awareness Project.
Restless legs syndrome, a condition in which you (or your partner's) legs twitch or shake during the night can also impact sleep quality. So can chronic pain, diabetes, heart disease, asthma and gastroesophageal reflux disease (GERD), to name a few. Medications can also impact sleep, as can various mental health disorders, according to the Cleveland Clinic.
"The take home message is that if you're not getting quality sleep, that means its time to see a sleep specialist to see what's going on," Dasgupta said.
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How to fall asleep faster
Now that your expectations are realistic -- it's not healthy to go out like a light, and you shouldn't worry if you need 10 to 20 minutes to fall asleep -- what about those nights when sleep is overly elusive?
Get up! "If you can't fall asleep after 20 minutes, get out of bed and go into another room where there is dim light, and do something calming until you feel drowsy again. The same applies when you wake in the night and can't fall back asleep," Dasgupta advises.
Robbins agrees: "Promise to never toss and turn -- commit to leaving your bed if, for whatever reason, you toss and turn at night and experience difficulty falling asleep."
Some people believe that it's just as refreshing to your body to lie in bed with eyes closed but not sleeping. That's just a pipe dream, Robbins said: "If we stay in bed, we'll start to associate the bed with insomnia."
Keep the bed sacred. Keeping your brain from seeing the bedroom as anything other than a place to sleep and have sex is how you train it to fall asleep when your head hits the pillow, Dasgupta said.
"You're much more likely to drift off quickly if your brain knows exactly what to expect when you enter the bedroom," he said.
That means don't work or watch television in bed, and don't make calls or check your cellphone. Blue lights from electronic devices tell our brains to wake up, not sleep.
Build a nest. Keep training your brain to expect sleep by nurturing the sleep process. Keep the room cool and dark. Science tells us that we sleep better in cooler temperatures of about 60 to 67 degrees Fahrenheit (15 to 20 degrees Celsius).
Ready a routine. Brush your teeth, take a relaxing warm bath or shower and then spend some time in dim light reading a book or listening to soothing music. You can try yoga or light stretches, but nothing that will rev you up. You're teaching your brain to wind down.
Go to bed and get up at the same time each day, even on weekends or your days off, the CDC advises. Before long