Crew rest is defined as the non-duty time before a flight duty period begins. It includes free time for meals, transportation and rest and must include an opportunity for 8 hours of uninterrupted sleep every 24 hour period. Crew day is defined as flights or work duties. As Gatordev said above, if you go over 18 hours of crew day (for flight/alert duty) , crew rest increases to 15 hours of continuous off-duty time. Most squadrons have a 10 or 12 hour crew rest rule in their SOP as well, to help honor the 8 hours' sleep rule. I've only been scheduled over 18 hours a few times (probably less than 10 in my career), usually due to double-pump flights followed by mission planning during workups, or super-long watchstanding on the Boat during workups or cruise.