Hustle
You can hustle for 1 hour without a problem. Hustling for a second hour in between sleep cycles deals 1 nonlethal damage to you, and each additional hour deals twice the damage taken during the previous hour of hustling. If you take any nonlethal damage from hustling, you become fatigued (see page 276). When you recover from this nonlethal damage, you also eliminate the fatigued condition.
Source
Core Rulebook pg. 258