"It is important to acquire deep sleep during the night because this stage of sleep has been shown to be more restorative for metabolic and cardiovascular function, as well as for learning and memory," said Zee.
A certain amount of relaxation must be acquired to reach the deeper stages of sleep. While sleep on a train or bus may allow for stage 1 or 2, being on public transportation makes many feel heightened anxiety, which would normally block the body's ability to reach later stages of sleep.
"Comfort levels come into play here," said Kohler. "A lot has to do whether we feel protected and safe. The mind can act as a guard that will try to keep you awake if it feels it needs to protect you, so a fragile or strange environment can reduce the ability to get a restful sleep on one's commute."