I've seen a trend in the type of players that play different times during the day.
(Note all times are in Eastern time, adjust for your time zone)
WEEKDAYS
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Evening from 6 pm to 1 am has a variety of players, but most are normal people that work 9-5 and want to relax. It depends on the table limit, most of the 9-5er's play $2/$4 and under. The competition is medium to hard
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Daytime from 9 am to 5 pm has many retiree's playing. The competition is easy to medium based on the table limit. I find that many play $0.50/$1.00 for some odd reason.
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Morning from 2 am to 8 am has night-owls. They can't sleep, or they may just feel like doing something rather than sleeping. Many play low stakes from $0.50/$1.00 and down. The competition is usually easy.
WEEKENDS
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Evening from 6 pm to 1 am has many good players. This is usually the busiest time for poker sites, and there is always action at the high-stakes tables. You have to earn your money with these guys. The competition varies a great deal from easy to extremely hard.
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Daytime from 9 am to 5 pm has better than average players, many pro's play online before they head to the B&M casino. You can find some easy fish to play, but for the most part, the competition is medium to hard.
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Morning from 2 am to 8 am is filled with party-animals and bachelors. The limits are usually $1/$2 and lower, and the competition is usually easy. You will find much more action is the mornings on weekends than you will on the weekdays. Also many college students will be on. The competition is from easy to medium.
This is how I feel the different times compare, feel free to adjust it

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