Here's a few things to try:
Increase the temperature setting on the Air con
Air conditioning units typically do two things: lower the ambient temperature, and remove humidity from the air. Depending on where you live the humidity can be a bigger factor in getting a good night's sleep than the temperature itself, as it exacerbates sweating and makes the air feel thicker.
If your aircon window unit is set to a higher temperature, it won't need to work as hard to maintain it, but it can still remove a lot of the humidity from the air and help you get a better night's sleep.
Clean the air filter
Look at your air conditioner's filter on a regular basis, especially throughout heavy use times. If the filter seems dirty after a month, clean it (or replace it if it's not a reusable one).
A dirty air filter is likely to slow air flow and make the unit work harder in order to keep you warm or cool — burning up energy and increasing the chances of maintenance issues down the line.
Have your air-con serviced
Air-cons need occasional maintenance especially if the unit is getting on in age. It's best to have this done professionally. The service person will generally check the refrigerant gas levels, test the thermostat and make sure all the internals are in good condition.
Compare the efficiency of your air-con with newer makes
Depending on the age of your unit, there may be more energy efficient options available on the market. Definitely double check the energy efficiency before making a purchase.
Depending on whether or not you own or rent these options may or may not apply, but consider the following:
- Check for air leakage - Not just under doors, but gaps in window frames, cornices, around vents or downlights, gaps in floorboards (they shrink over the years). This site gives a good overview of the sorts of places to check and gives good guides for detecting and mitigating air leaks.
- Insulate - Add Roof batting, Underfloor (or undercarpet) insulation and if possible, wall insulation. Insulation works double duty - helping to maintain warmth in winter and cold in summer.
- Install Double-glazed windows - basically the same thing as insulation but for your windows, one of the primary ways heat (or cold) escapes a room
- Door Seals - look at installing proper door seals for doors with gaps.