Havn't been to SDF in a long time but both SnapFitness and Anytime Fitness show locations within 3 mi. of the airport. Might not be close to Highlands but lots of 24/7 clubs all over the country.
Snap is 1 train stop from MSP and Anytime is the first bus stop from main terminal at MSP, for example.
Find a Snap Fitness gym
Anytime Fitness Locations