<unk>  is a condition in which when a person transitions from a supine (lying down) position to standing upright there is an insufficient amount of blood that returns back to the heart due to poor circulation. This lack of blood traveling back to the heart causes an immediate decrease in blood pressure and an increase in heart rate which could lead to lightheadedness, fainting, and extreme fatigue.
Postural orthostatic tachycardia syndrome
Heat syncope
Vasovagal syncope
Neurocardiogenic syncope
Defecation syncope
Micturation syncope
Serotonin syndrome
Cholinergic crisis