According to a small 2018 study published in Chronobiology International, sleeping upright may place the body in a state of greater physiological arousal, subsequently making sleep more difficult. The researchers explained that sleeping upright suppresses the body’s “rest and digest” activity, which is regulated by the parasympathetic nervous system. Instead, it activates the body’s sympathetic nervous system, which is associated with stress and heightened awareness such as when we’re in “fight-or-flight” mode. This was evidenced by the fact that out of six study participants, sleeping for four hours in an upright seat decreased total sleep time by 29%, slow-wave sleep by 30%, and REM sleep by 79% in comparison to sleeping in a chair that reclined completely flat.
Another thing that can happen when we sleep sitting up is that our body may become more susceptible to blood clots, reports Sleep Advisor. Extended time spent sitting can lead to the development of deep vein thrombosis, in which blood clots form in the legs. If these blood clots make their way to other parts of the body, such as our respiratory tract, it can be life-threatening. Talk to your physician if you experience symptoms of pain, swelling, skin discoloration, or skin that feels warm to the touch.