Are pilots authorised to use sleeping pills to rest before long haul flight that departs at night hours? This question comes in mind as I have flown Turkish Airlines flights that depart at 2.30 at night arriving in the morning with the duration of the flight of about 8 hours. Since the pilot or the co-pilot have to be awake (I suppose, at least one of the 2..) during the flight they will have to get good rest during the day. Sleeping during the day is quite difficult and not natural and this is where my question comes from. If not, what remedies do they use?
Short answer: Sleep aids cannot be used this way in the US. FAA regulates the use of such medications with a delay:
More on medication advice to medical examiners: Pharmaceuticals (Therapeutic Medications) - Do Not Issue - Do Not Fly.
What you are referring to has got the scientific name of circadian rhythm (circa = around, diēs = day), which is the subject of an article at SkyBrary: Circadian Rhythms. About the night flights:
Night operations create a host of problems for flight crews. The primary problem is having to work efficiently and safely at a point in time when the work requirements are not in synchrony with circadian rhythms. Under worst-case conditions, crew members must perform demanding tasks during the early morning hours (2 am to 6 am) when their biological functions and performance efficiency are at their lowest level. This problem cannot be quickly solved by adaptation of the biological clock. Complete adjustment to night work requires at least 21 night shifts in a row with no days off. Adjustment of the biological clock does not even commence until about 10 days after a shift change.
Long article about Fatigue Countermeasures in Aviation, about zaleplon:
This misalignment of the sleep/wake cycle and endogenous circadian rhythms could be the result of shift lag, jet lag, or attempting sleep at other than the habitual bedtime or at the second circadian dip in the afternoon. For such circumstances, the U.S. Air Force and Army have approved the limited use of temazepam, zolpidem, and zaleplon. These hypnotics can optimize the quality of crew rest in circumstances where sleep is possible, but difficult to obtain.
As additional information about sleep management inflight: On a 8-hour flight, usually there will be 3 pilots, and each one will be able to sleep (outside the flight deck).
Usually, pilots (and ATC staff) are not allowed to take any sleeping pills during duty. In case they had taken a sleep aid, they are not allowed to perform the (pilot or ATC) duties for a specified time depending on the drug. According to FAA guidance:
Because of the potential for impairment, we require a minimum wait time between the last dose of a sleep aid and performing pilot or ATCS duties. This wait time is based on the pharmacologic elimination half life of the drug (half life is the time it takes to clear half of the absorbed dose from the body). The minimum required wait time after the last dose of a sleep aid is 5-times the maximum elimination half life.
However, the pilots are allowed to use sleep aids occasionally for reasons such as circadian rhythm disruption in commercial air operations provided they don't operate aircraft as noted above.
Pilots use a number of ways to cope of with sleep deprivation, especially in long haul flights.
A number of non-US carriers (like Air Canada, Air New Zealand, British Airways, Emirates, Finnair, Lufthansa, Swiss, Qantas etc) allow in-seat cockpit naps under certain conditions, though FAA has not allowed this still.
Also, ultra-long flights allow for in-flight bunk sleep periods for (both the) pilots.
Aircrew are allowed strict rest periods between flights by regulation.
Pilots alternate flying duties so as to reduce fatigue.