For a period of about 3 months, saving for a GOOD month off, I was working 9-9 with 2 half-hour breaks per day, 6 days a week, I survived on 4 hrs daily, I did, however take loadsa "no-doz" (if u guys dont get them over there, it's like concentrated caffeine tabs) when I eventually took my "holiday" I was like fucked up for 2 weeks....
Generally, I have about 5.5 to 7 hrs daily, howsat for a nice boooring answer?

i get headaches also but mine r from like sun light and really bright lights. i have epilepsy and bright stuff like the sun tend to cause seizures w/ me at times but only a fews times its happend. so if i got out i usually have a visor on or sunglasses to keep the sun out