After Wanda Cwiecek began treatment for multiple myeloma, a cancer of the blood, she had trouble sleeping and felt emotionally drained. She would get home from work so tired she immediately needed to lie down.

Ms. Cwiecek, who received chemotherapy in 2014 and a stem-cell transplant for her cancer, last year took part in one of a series of randomized clinical trials at the Icahn School of Medicine at Mount Sinai to test whether regular exposure to bright white light could reduce the extreme fatigue and feelings of depression that affect many cancer patients.

For 30 minutes every morning over four weeks, Ms. Cwiecek, a 64-year-old paralegal in New York City, sat near a special light box that emitted an intense white light. She would usually have coffee and watch TV news during the sessions. She quickly felt improvements. “Life got easier,” she says. She began to sleep better at night and was less tired during the day. “I felt happier. I had more energy with the light,” she says.

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Overall, 54 cancer patients participated in the latest trial—about half were exposed to bright white light and the rest, a control group, were given a dim red light to use. One of the lead researchers, Heiddis Valdimarsdottir, reported preliminary results at the annual meeting of the American Psychosomatic Society in March. Patients exposed to the white light had significant improvements in relieving symptoms of depression, while the control group had no real changes in their condition, says Dr. Valdimarsdottir, an assistant professor of oncological sciences at Mount Sinai who is also with the University of Reykjavik in Iceland.

An earlier study the researchers conducted found light therapy, known as systematic light exposure, also helped patients feel less fatigued.

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