system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How do we sometimes feel tired when we haven’t moved or exhausted ourselves physically?

Like sitting all day at the computer doing a project or during a flight sometimes when you land you feel super tired??
gpt
Fatigue does not only occur due to physical exhaustion. It can come from vitamin deficiencies, dehydration, stress, or mental illnesses like depression. In case of deficiencies, your body does not have all it needs to function with vitality. If you are dehydrated, you get headaches and lower blood pressure because your body does not have enough water to make you feel vital, just enough to keep the basic stuff running, like your lungs and heart. Stress already releases hormones into your body that keep you jittery and wear you out... we should not be on constant alert to fight or flee when our greatest danger at some moment is being worried about a work meeting.