You are right that your Epiphyllum could use a rest from flowering to restore its vigor. It is possible that you are signalling it to bloom repeatedly through temperature, water, and/ or nutrition. Your Epiphyllum needs to experience "seasons" of fairly pronounced differences in care to stay healthy over the years. Let me run through a recommended care routine so that you can compare that to what you are doing and make any necessary adjustments.

These plants prefer a medium source of light year-round and will eventually die if exposed to prolonged, direct sun. Morning sun only is ideal, though they will survive less than ideal light exposures. Rotating the plant is beneficial but not essential. If you are able to change from the south-facing window to an eastern window, or provide some filtering of light in the current position, that would be an improvement. It is possible that there are trees or obstructions outside your window that are already filtering the light, so you should evaluate that.

Temperature, water and feeding change with the time of year. Here is a typical approximate schedule. Your plant will indicate to you the timing for moving from one stage to another through completion of flowering, bud setting and flower formation.

• December, January and February: Resting Period - keep your plant cool, between 45 and 50 degrees. Water infrequently, only enough to keep soil barely moist for a period of about six weeks. Do not overwater; if your plant begins to turn yellow at the tips, you are giving it too much water and causing the cellular structure to rupture.
• March and April: Pre-flowering Period - continue with cool temperatures and reduced watering, letting the top ½ inch of soil dry out between waterings, until you see buds form. When buds form, increase water and temperature and begin to feed plant. Use a tomato-type fertilizer every two weeks through the flowering period and then stop.
• May to mid-September: Flowering Period - Water frequently, keeping the soil consistently moist but not standing in water run-off. Increase temperature to a minimum of 60 degrees and up to 70 degrees. Use a tomato-type fertilizer every two weeks through the flowering period and then stop. If you ever have an option to move the plant outdoors in a protected position during the summer, that will help to strengthen the stems.
• Mid-September - November: Growing Season - Maintain the flowering period care, tapering off as you reach the resting period, but stop feeding once flowering has finished.

I think that should give you a pretty good idea of the ideal care for your plant and then you can decide how best to achieve it in your home. Epiphyllum cacti like to be pot bound, so don't repot unless absolutely necessary. When you do, use the same size pot, mixing one part perlite (for drainage) with one part potting soil, one part peat and one part coarse orchid bark (for structure and nutrition) to create an excellent soil for your jungle cacti. You will notice that while this mix is fast draining, it will hold more moisture than a desert cactus mix which mimics its native habitat.

Courtesy of NYBG Plant Information Service