Link between sleep, breast cancer? back now with health news. new findings about certain sleep habits. estimated quarter million women inl the u.s. will be diagnosed with breast cancer this year. dr. nancy snyderman reports. mary ellen davis remembers that june day two years ago. she said you are not going to hear anything after what i tell you. you have breast cancer . davis had the most aggressive form of the disease. she was 44. my sister came over to my mom's and we all just cried. like davis , she had no history of the disease. which is largely associated with risk factors like aging and obesity. now other risk factors like working overnight shifts which davis did for two years. surprisingly, those most at risk weren't the women who worked the night shift all the time. women who worked four and a half years of night shift with an average of three days in their workweek where they were working the night shift they were at higher risk. the risk was higher in women who started working the shifts before they had children. one reason for the association between irregular sleep and risk for breast answer could be the lack of a hormone called melatonin which is produced during sleep. it helps to break down molecules which daus dna to be damaged. it doesn't mean women stop do doing night work . it means the longer that you are in that process the longer the risk may be to you. this study adds to the list of lifestyle factors we can control. for younger women to change their lifestyle or decrease their risk factors . i would hate to see anybody else go through it. dr. nancy snyder man nbc news, new york.