Expect variation. January in Orlando can be hit or miss weather-wise.
Most days for us were in the high 60's, low 70's our last two trips -- T-shirt & shorts weather for a Northerner. Sometimes the nights were quite cool at about 50 degrees & other nights were quite balmy. Again, if you're from the North, jeans & sweatshirts at night or a light to med-weight jacket should be fine for those occassional cool nights. Bear in mind, the locals wear winter hats, coats & gloves when it drops to 60 degrees (which my kids found amusing). Also, all the gift shops will put out hats & gloves for sale at about that temp too. Meanwhile, we were still using the pools at 60 degrees and having locals looks at us like we were crazy. Family rule: if you don't see your breath cloud in front of you, its alright for swimming!