I often have small episodes, lasting 30-45 seconds, of tunnel vision, tingling in arms and legs, cold sensation sweeping my body, and feeling faint. I sit down and slowly lower my head between my legs for about 30 seconds then slowly sit back up and it's usually gone except the cold sensation sticks around for a few minutes. It happened again tonight, I should add that I have been up for about 40 hours at this point. I pulled an all nighter last night doing school work and I am up late tonight doing more school work. This is the first time I have ever stayed awake this long and this has happened on multiple other occasions when I was well rested..  A majority of the time it happens while I'm in the shower, I have to sit down or I feel like I'm going to pass out. I have no current or historical health issues and don't take any medications. Any ideas would be greatly appreciated!