From a neurological perspective, there's no such thing as free will.
I'm not even joking. The people who work most intimately with the brain -- the neurobiologists, the cognitive scientists -- tend to think "there's not a whole lot of free will out there, and if there is, it's in the least interesting places."  If you read about the workings of the brain, it discusses the frontal lobes, the "executive body" of the brain which makes decisions from the evidence presented to it.  If you read about the process by which brains tend to make decisions, it's a combination of a darwinian "war of ideas" (literally, ideas fight it out in the area for control of the cortex) and pattern recognition -- the brain picks out patterns that it recognizes from a sea of information and will see those patterns until something better makes it pry those patterns loose. 
Philosophically speaking, we have free will in that we can choose what we want to do. The catch here is that we can only choose to do things that we want to do; we choose to do that which is in our nature. And we don't choose our nature. We don't decide to be gay or straight. We don't decide that we're extraverted or introverted, timid or brave, anxious or confident. We simply are these things. And while making decisions against our nature may enable us to become more comfortable outside our limits, it's something that we have to push ourselves against, again and again, before we can say our boundaries have shifted.
The fact that drugs can change the way we think is proof that our idea of free will is idealistic. Forget prozac and ritalin. We know that you can make your boundaries and the way that you think with enough tweaks to serotonin. But we alter our mental states daily with alcohol and caffeine. Our idea of what we choose freely is moderated by "I was drunk" or "I was on my fifth espresso." If it's possible to be "someone else" so easily and so casually, does it make sense to talk about an ideal, unadulterated self at all? When you talk about yourself acting of your own free will, you're talking about you with a healthy unperturbed brain.
This also means that "free will" is not a binary switch. It's more of a continuum. At one side of the spectrum, you're focused and aware in the moment, choosing your words, recognizing new options, picking things up. You know that everything you do right now matters. At the other side of the spectrum, you're running on autopilot. You may be drunk, tired, caught in the past, or recognizing a false pattern. You may have your buttons pushed. But you're predictable. You don't have any creativity or self awareness. You can't learn.
So what, in the brain, causes free will? It's the same brain, right? What causes the brain to be worn out, or to be fresh?
There's a series of experiments done on monkeys that tells us what makes monkeys and mice able to learn, whether it's finding their way through a maze or finding a raised platform to avoid being dunked in water. They find that when animals are raised in a stimulating environment, they do much better at solving puzzles. When they looked carefully at the brains of those animals, the researchers found that their brains show evidence of neurogenesis; new brain cells were found in the hippocampus. Meanwhile, animals that were not raised in a stimulating environment (or worse, were raised in a stressful one), showed little or no neurogenesis. 
If you abstract the research and apply it to humans, it's reasonable (although complete speculation) to argue that human beings will show also evidence of neurogenesis given a stimulating environment. Certainly it's known that humans raised in a stimulating environment do much better than humans raised in an unstimulating environment, and reducing or eliminating stimulus to humans is considered to be a form of torture.
The interesting thing about tying healthy brain function to neurogenesis is that widely known mental practices suddenly reveal themselves to have a biological basis. Sleeping eight hours a day. Regular exercise. Play.
The function of play in neurogenesis is especially interesting, because play involves the ability to recognize patterns and make quick decisions without consequence to the "real world". It gives the brain room to "limber up" and build up extra neurons that can be used later. Those neurons give us the ability to see things in a new way. It's freeing. You don't know who or what you'll be at the end of the game, but you know you'll be someone else. It's an enjoyable sensation and an end in itself. People who have brains like this have the most free will: they can make decisions taking multiple factors into account with more mental resources available to them.
This also explains the biological aspect to being burnt out -- work too hard, or too long, and you'll use up the store of fresh neurons in your head. People who work too hard or too long are unable to see new patterns or respond appropriately to unexpected stimuli. Their free will has been compromised. They don't just look like being zombies; they are zombies.
I always thought about free will as important, weighty stuff, worthy of a philosopher. But it turns out that free will comes into being when we are at play. It's only after that that we get back to work with our fresh neurons, and by then it's already set in.
Free will is the ability to be silly.