While I'd rather ponder the possibilities of a planet suspended in the Lagrangian point between the stars and mechanisms that lead to such configuration, here's what I think about sleep.
It is not necessary for humans to sleep every night, 8 hours straight, or even regularly(though regularity is beneficial). While sleep deprivation does affect people very much like narcotic substances (diminished motor control, grogginess, up to even hallucinations), the need to sleep does not come from the changes in light levels. I've personally experimented and realized that about 28-hour cycle suits me better than the 24-hour locked-to-day cycle. During the time I didn't see the sky to assess the time of day for long stretches of time, so my body forgot the regular cycle and fell into what was "required", rather than timed by the setting and rising of the sun.
There are several different sleeping patterns you can employ to alter your cycles, some claiming to be more efficient(requiring less sleep per 24hr cycle) than the solid "8 hours at a time" we've grown to hold as the status quo. Here's a blog of a person switching to 20 minute naps for every 4 hours, one of the most extreme polyphasic sleep cycles that maximizes the time you are awake per day. 551
As he explains it too, while you might get more active hours to your day with a "powernapping" sleep schedule, the downfall is that those smaller windows of sleep need to be extremely efficient. After about two weeks of extreme sleep deprivation(not reaching REM sleep during the naps) his body adapted and he'd be able to get to REM stages within minutes of falling asleep. The final post(A year after he ended the experiment) details some of these pros and cons.
Should you have no clear cycle of day and night, then time would simply not measured by the changes of light levels, but perhaps by some contraption that had a really reliable interval in it's action. This would also perhaps create a society where people are awake at whatever hours suite them, rather than the uniformed cycles most people on earth follow. A polyphasic sleep cycle might easily be the norm in such societies, since there would be no clear distinction between times of "day". 551