On a 13-hour flight all shades were ordered closed (entire flight—not cool), same airline on a 3-hour route, same thing.
Both flights took off in the morning, around 8 AM.
Only reason I can think of is to keep the cabin cool, lower the demand on the engine bleed, and burn less fuel?
I ruled out the minimize jet-lag reason as sleeping masks were handed out and same thing happened on the shorter flight.
It isn't due to "lowering the demand on the engine bleed" because the air conditioning is done as part of the standard pressurization of the cabin - the air is bled from the engine, cooled to a very low temperature and then heated to the desired temperature for the cabin, so shutting the blinds wouldn't affect that because the air is still cooled and then reheated.
It is purely for jet-lag and sleeping issues - a single blind open in a darkened cabin can disrupt even those people wearing the eye masks (those things arent perfect) and can disrupt people who have woken to go to the toilet during the cabins "sleep period" (think how horrible it is if you get up in the middle of the night and turn on a bright light - its easier to get back to sleep if you keep lighting muted throughout your activity than it is otherwise).
It's to let people sleep and to minimize jet lag. Sunlight messes with your circadian rhythm. If a passenger needs light, they can turn on their personal seat light.
Ever try to watch the in-flight movie with the window shades open?
Most likely the shades were ordered shut to enhance the viewing quality for the majority of the passengers. Keeping the pax in their seats watching the movie reduces the workload for the cabin attendants.
The reason is not to save fuel.
There are a couple of primary reasons for requiring window shades to be closed during flight:
Nearly all East-West long-haul flights (i.e. those crossing lots of time zones) will require window shades to be closed and will turn off cabin lighting for most of the duration of the flight (usually for the entire time between meal services.) This is because almost every passenger wants to sleep and many of them need to sleep on different schedules.
People on short business trips often want to remain on or close to their home time zone in order to reduce the effects of jet lag. People on longer trips usually want to adjust to their destination's time. As a result, these people will need to sleep at different times, so the lights are kept off - and window shades kept closed - for the vast majority of the flight in order to accommodate whatever sleep needs people may have.
An eye mask can certainly help, but some people don't like wearing them and also people need to be able to get up briefly to use the lav or grab a brief snack and then want to doze off back to sleep. This will be much easier if the cabin is kept dark.
Even on short flights, at certain times of day and when traveling certain directions, a lot of sunlight can enter the cabin. This can make the cabin uncomfortably hot, especially for people near the windows and/or in direct sunlight. Additionally, the light can be very bright if it's shining in directly through the windows. Even on short-to-medium length daytime flights, some passengers will want to nap, but even those who aren't napping generally don't want the sun shining directly into their eyes. Note that, while the sun from your window may not be shining directly onto your face, it very well could be shining directly into the face of other passengers on or near your row, even across the aisle, depending on time of day (and year) and flight direction.
There's actually enough UV to cause sunburns on directly exposed skin, since airliners fly above the majority of Earth's atmosphere (the majority of the gas in the atmosphere, that is, not a majority of the height of the atmosphere.) There's not nearly as much atmospheric absorption or reflection of the sun's light at 30,000-40,000 ft as there is at the surface, so direct exposure for extended periods, in addition to being annoying, can be harmful.
While this reason isn't as important as the other two, a third more minor reason for closing the shades is so that people can see their IFE screens. The screens can be difficult to see if there's sunlight shining on them.
