I assume that you are wanting to save power. If you have motors that drive mirrors, the motors will use thousands of times as much power as the PIC so shutting them off, which only needs code and doesn't need sleep mode will save you most of your power.
Using a PIC16LF877A, with a low supply voltage, will also save a good fraction of the power used. Running the processor at a slow clock speed cuts the power right down.
If you are going to use the A/D converter, it will run in sleep mode.
However, there is very little point in actually sending the processor to sleep while doing an A/D conversion. The conversion takes about 40 µs and you have a similar acquisition time before that.
A good way to save power is to set a timer, running on a 32.768 kHz watch crystal. Send the processor to sleep until the timer times out. Then turn on the A/D converter, do the conversion, and if it is still dark, go back to sleep. You can connect a watch crystal to timer1 on a PIC16LF877A, and use that to wake up the processor. The RC oscillator is best for the processor in an application like that, because it starts up quickly.
You are only looking for changes in daylight, so you could set the timer for several minutes. You would do that on a PIC16LF877A by only taking an A/D reading after being woken up a number of times.
If you are really keen to save processor power, other processors allow you to change the clock speed as well as going to sleep.
Alternatively, feed the PIC from solar cells. It will just stop at night, and restart in the morning. You loose any data in the registers but the program is safe, as is what is in EEPROM.