Brake Light Coming on in Cold AM, Goes Off When Car Warms Up?

I have a 1993 Mazda Protege that I bought new off the lot and have carefully maintained for 15 yrs. It has 150,000 mi on it. Since Monday, the brake light has been coming on in the recently chilly mornings but it goes off as soon as the car is sufficiently warm (approx 15 min). It does not come on in the afternoons when temps are 80-90F. They are not ABS. What could be causing this? Is it an immediate cause for concern? Many many thanks for any insight on this.