I assume this is not to be confused with 'micro-napping', something which I and many of my colleagues who fly night freight often experience. A change of activity, suck a sweet, get out of the seat for a bit etc. often does the trick. My sympathy is with you over this as you do not have the luxury of CRM.
A couple of minutes is a relatively long time though. At which point is this considered narcolepsy? Are those of us who constantly learn tricks to fight sleep deprivation, at risk of this developing?