Allergy issues due to mattress

Hey, I’m here for a good suggestion from my forum mates.
My niece, Samanta, always has an interruption in her sleep and shows mood swings for the last year. She is very disturbed in her class due to the tiredness.
She lost her concentration on her studies, and she does not involve in any extracurricular activities in her school. She was a good singer in the church choir before. Now everything changed worst. My sister is always worried about what happened to her daughter. My brother in law decided to take his daughter to the psychologist.
After a long discussion with her, the doctor found that all happened because of her sleep interruption. He advised us to change the mattress. Doctor himself suggested Stearns and Foster beds for better results.
We are in a dilemma. We are now using an old mattress. Can anybody give me a suggestion on the mattress mentioned above? Please guys it’s very urgent!!!