Thousands of registered nurses from eight Bay Area Sutter Health-managed hospitals are striking Tuesday and are planning to participate in rallies to protest a proposed decrease in their employer-covered health care expenses. It is the third strike in seven months by members of the California Nurses Association over contracts that have been under negotiation for nearly a year. Some 4,500 nurses went on strike 7 a.m. today, and plan to strike a full 24 hours, according to the nurses association. Discussions have reached an impasse because nurses say Sutter is demanding that they forgo paid sick days and pay more toward their healthcare, among other requested concessions.