We all take them. But we all know how easily a 10 minute study break turns into a 1 hour waste of time. Or longer. Trust me I do it all the time and I always regret it later.
Last week I told myself I'm going to watch the first 10 minutes of the 7th Harry Potter movie while I ate dinner. I ended up staying up til almost midnight to finish the movie and by that point I was too tired and had to go to bed to get up early to do the work I had neglected. What did I learn? 1) You cannot "just watch 10 minutes" of anything Harry Potter. 2) That movie is 130 minutes long, longer if you pause in the middle to see what actors/actresses are playing in now on imdb. 3) The enjoyment was not worth the colossal waste of time my night turned out to be.
So I give you this:
Nassim's list of appropriate study breaks that won't sabotage you:
Book: Completely not engineering (or whatever your major is) related. My current choice is Void by Rhiannon Lassitor. I've read it before so it's easier to stop and its a whole lot more exciting than my before-finals reality. 1 break = 1 chapter. Stick with that. Exercise self control and don't go beyond it.
Workout: A quick workout gets your blood going and gets your brain to focus once again. I'm not saying run a marathon, you can get in your full workout either early in the morning to wake yourself up or late at night when you just can't study any longer. I usually go for my run earlier then save my weight training or yoga for these times. Set a goal for these or just set a timer. Some ones I use are goals like 20 reptile pushups/60 sumo squats, or my 8x1 (8 exercises, 1 minute each) abs workout, or I run through a short Yoga circuit.
Make a Snack: Don't stand in front of the fridge and drool and wish someone would make you a sandwich. Actually make one and eat it. Don't read, don't sit around on facebook, just eat and go back to work. Smoothies are also fun. (I need to use this more, my breakfast time has become my catch-up-on-cop-shows time. Rookie Blue is back for Season 2!).
Draw: Doodle, you know how it works. It's how I made it through AP English without falling asleep.
Write: preferably by hand, away from the computer. Write whatever floats your boat.
Clean: I am not a clean person. I keep things sanitary (I don't like bugs) but my room is usually at a state where it looks like a hurricane hit it. Putting a few things away at a time during breaks helps me clear my mind, keep me from injuring myself again tripping over something I can't see, and it keeps my roomate from strangling me in my sleep :) All things that make me happy. I also usually end up finding things I've been looking for.
You get the idea. The most important point is self control, it become easier with time. I have found for instance that I cannot play video games as a study break. I have no sense of self control when it comes to them and sometimes "I'll just stop at the next save point" lasts several hours. If I die before I get there (how is there always a boss battle before a save point) I feel it's unjust and I must keep trying until I get there.
If you really feel you need it, use a timer. I just find them irritating when I'm trying to focus on something actually enjoyable.