I work all the time, daytime, evenings, nighttime. Sometimes I come here for a break, sometimes because I need a solution from the serious barkeeps and gurus.
There is no pattern.
Being a software developer is different from running a cash register or flipping burgers. Productivity can't be measured strictly by hour and minute. By and large, developers are professional enough to maintain their productivity without companies having to resort to (probably futile) means like restricting internet access. Though I've occasionally been guilty of spending too much work time here or other sites, usually my self-discipline is enough, and definitely more effective than imposed discipline. It's like herding cats, right?
Self discipline is over rated. All you need is a boss with a whip and hobbed-nailed boots, and the usual deadlines for software projects (i.e. impossibly short) and you won't waste any time.
There is a reason that Google and other startups have free food available 24 hours a day. They expect you to need it.
To get a wish, you need a genie. To get a genie, you need a lamp. To get a lamp, you need a tiny ad: