Many of you traveling home to visit friends and family over the holidays will encounter the eventual need to sleep on an airplane, train, or bus, or in a car … while sitting up. Have you ever wondered why it's so much more difficult to get a good sleep in the upright position as opposed to lying flat in bed?

In order to understand it, you first need a quick primer of your sleep cycles: The human brain cycles through five phases of sleep when your head hits the pillow, the fifth of which is called rapid eye movement (REM) sleep. REM only makes up about 25 percent of your sleep cycle, and occurs approximately 70 to 90 minutes after you fall asleep, but you enter REM sleep several times during the night. During REM, the brain sends signals to the spinal cord to create a temporary paralysis of your muscles, which causes you to lose muscle tone. 

"Usually during REM sleep, other than eye movements, our voluntary muscles are paralyzed," Dr. Neil Kline, an internist and sleep disorder physician and CEO of the American Sleep Association, tells Mental Floss. "We likely evolved this disconnect during REM sleep in order to prevent injury to ourselves." For individuals who suffer from REM sleep behavior disorder (RBD), “this disconnect is faulty," Kline says. "People with RBD act out their dreams, and often injure themselves and others."

The partial paralysis and loss of muscle tone make holding the upright posture of a straight back and neck difficult. It may be why your seatmate tilts sleepily into your personal space, snoring on your shoulder, and why sleeping on a plane is just hard to do comfortably.

Despite this difficulty, some Buddhist monks purposely sleep sitting up as part of their meditation practice for up to five hours per night. They sit in specially designed chairs with hard, firm backs and proper cushioning, and do this as part of a goal to be in meditation as many hours of the day as possible. Your goal may be just an hour of snoozing between takeoff and landing.

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