well, there's a lot of factors to consider including:

a) is your electricity cheaper at night to "purchase"?
b) when do you like to do your maintenance (adding chemicals..you'll want your pump circulating whatever you add and circulating long enough to cycle them through.)
c) if you're using a heater, when is your swim time?

in our case, i use a 8pm to 2am and 8am to 2pm cycle.

i get home from work about 3:30 as do the kids so i might flip the pump on and in turn the heater this time of year, go do my treadmilling and the pool is gaining 1-2 degrees still being in the sun and with the help of the heater. go for my swim, shut the system off about 5pm.

i do any chemical adjustments post 8pm before i go to bed so i have a good cycle. i go to work 5-5:30am so it's not feasible for me to be around for the 8am pump startup.

i have a solar cover so i do want a good cycle during the day to keep the chlorine circulated..it can get pretty warm underneath the cover and i could be concerned about algae growth with a still pool under a solar cover especially during the hot summer days.