Answer: Cyperus often seems to suffer when brought indoors for the winter because the plant was accustomed to higher light and higher humidity outside as well as, often, warmer temperatures.
Indoor heating makes the air very dry, and this can also encourage spider mited, so check the plant carefully for any signs of webbing or stippling patterns on the foliage. If you find some, they can be controlled with frequent sprays of water and/or insecticidal soap applied according to the instructions.
To help the plant winter more happily, provide it as much light and humidity as possible and keep the pot in a saucer of water so the soil stays very moist. Trim away any browned stems as they occur.
In spring, the plant will naturally begin to rejuvenate in response to the longer days. It may also benefit from being repotted into a larger pot, or being divided and returned to its original sized pot.
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