Sounds low. I've been getting about 39mpg on average lately in my Corolla (5A-FE 1.5L 5-speed). That's with about 90% city driving and 10% highway driving. Admittedly, that's driving like a bit of a grandma, but typically I'd see no less than 34mpg when driving 'normally'. You'll use more fuel at night due to the cooler air temperature, but the fuel mileage shouldn't be that low, unless you have a really heavy foot, a lot of weight in the car or drive everywhere with roof racks.
Start by checking how clean your air filter is and how inflated your tires are. Also try changing the spark plugs, oil and leads. Have a look at the temperature gauge next time you're driving the car, it should sit on half or just under. If it's lower than that then there's a chance that your thermostat needs replacing, which can play a big role when it comes to fuel consumption.
There are a lot of other factors which could contribute to the poor fuel mileage, such as dirty or leaking injectors, vacuum leaks, exhaust leaks, sticking EGR valve, incorrect timing, worn valve guides, bad fuel pressure regulator or fuel filter, clogged PCV valve etc etc. Driving with the windows down at highway speeds can have an affect on the fuel economy too. Start by checking the simple things first and go from there :thumbsup: