At last year's Bahrain GP, track temperatures dropped from over 47 degrees in FP1 to 35 degrees in FP2. The following day, FP3 took place in 39-degree heat, but this figure fell to 30 degrees for Qualifying and it was the same for the race on Sunday too.
That's a drop of around 10 degrees (or more) between the daylight sessions and the twilight sessions. This can not only alter the balance of the car, but it also changes how the tyre is working, because it can move the Pirelli rubber out of its operating window.
Cooling and the brakes are also impacted by this temperature shift. Some teams may even use different cooling specifications in the daylight sessions, before switching over for the twilight running. It's helpful in some ways, to find out how the cooling handles the heat, but this alters the aerodynamics of the car.
Therefore, because of how the conditions impact the cars and the tyres, it's very tricky for teams and drivers to work on car set-up and balance in these two sessions. Things change so drastically for the sessions that really matter, so FP2 is the only real chance to hone the set-up and find the sweet spot.