First let me say that 8 hours is plenty, and it's natural that you would become fatigued after putting in a full day hitting the books. Don't push yourself - that leads to exhaustion, both mental and physical, and is counter-productive. Instead, find ways to schedule your time efficiently so that you don't need to put in more than 8 hours per day.
That said, there are a few things you might try, and they mostly involve lifestyle changes. Take a look at your sleep schedule - how late do you stay up, how early do you rise, and when are you more effective in your studies? Perhaps "early to bed, early to rise" might be the right plan for you, with LSAT study over breakfast, school during the day, and then more LSAT in the evening? Or are you like my son (and my wife, actually) and unable to think all that clearly until later in the day? If so, then get your sleep when you need it, in the mornings, and put in later hours with the books open after a healthy dinner. Make sure you are getting enough sleep overall, whenever it is that you need it most.
Look at your diet, and consider making small changes there - more leafy greens and lean proteins, less ramen and fast food, perhaps? Talk to your doctor, nutritionist, personal trainer, etc. to see if you can make some improvements in what you eat and also when you eat it (some folks say small meals throughout the day, others say breakfast is most important and nothing after dinner - I'm no pro here, so get professional advice from a trusted source).
Are you getting exercise? A regular routine of physical activity helps stimulate the brain as well as the muscles, and it helps build stamina and improve energy. When you take breaks from studying, it might be better to go for a walk, bike ride or swim than to veg out and watch a movie. Walk in the morning and/or before bed, maybe get others to do it with you. Part of this test is physical, after all - it requires stamina to sit for 4 hours and not grow fatigued, and improved circulation and muscle tone can only help. I'm not suggesting that this is a good time to take up power-lifting or to prep for a marathon, but take whatever you are doing now and dial it up a notch. If you aren't getting any exercise then start getting some, whenever you can. Park at the opposite side of campus, or on the far side of the parking lot when you go shopping; take the stairs instead of the elevator; even stand and stretch every hour while studying.
What I would NOT do is rely on temporary or artificial means of increasing energy. A 5 Hour Energy shot or a Monster might be great once in a while, but it's no substitute for healthy living (including appropriate rest). Not a coffee drinker? Don't start now. If you are a tobacco smoker, and if you are taking the test in February, this might not be the wisest time to quit, but it would be a great time to cut back. Quit right after the test, though.
Good luck! Take good care of yourself!
Adam M. Tyson
PowerScore LSAT, GRE, ACT and SAT Instructor
Follow me on Twitter at
https://twitter.com/LSATadam