Clearly it involves gaining levels of fatigue somewhere, until the character passes out, but are there rules for this in any of the 40k rpg books?
Rules for going without sleep?
The closest thing I can find on the subject is under the Legion Power Armor Subsystems on page 176 of BC. For every two weeks a character with the Nutrient Recycling subsystem takes a Tougness Test at -10 or suffers one level of fatigue. Sleep should be similar, though with a much shorter interval, perhaps only 24 hours. Additionally or to replace fatigue you can add negative modifiers to Perception based tests.