A nap is a period of sleep lasting between 20 minutes and 2 hours. It plays a fundamental role in reducing sleep insufficiency. After a normal night’s sleep, drowsiness has a peak in the middle of the afternoon and another peak at night. The period of least drowsiness is located between these two peaks. Napping early in the afternoon is a manifestation of the spontaneous two-phase character of sleep common to all individuals. The flight crew should alternate their rest and activities, including cockpit napping. If 2-pilot crew synchronize their activities along the flight, they tend to be active at the same moment and consequently to be tired at the same moment. Therefore, members of the flight crew are suggested to desynchronize their activities through alternating passive and active flight monitoring to avoid simultaneous decreases of their alertness. In the case of extreme sleepiness, an appropriate napping strategy should be adopted by each of them. Naps of around 20 minutes were taken during flight and planned in order not to disturb the regular flight progress; they had to be taken until one hour before descent at the latest. Studies have shown that: - pilots, while resting at their seats, can quickly obtain short sleep periods of good quality; - naps increase performance as assessed by reaction time tests; - pilots of the nap group had 5 times less drowsiness episodes than the control group, indicating the efficiency of these naps in maintaining alertness during flight.4. What is the meaning of synchronize in the second paragraph?