Heart attacks that occur at night are less severe than those that strike during the day. A new study from the Centro Nacional de Investigaciones Cardiovasculares (CNIC) explains why. Published in the Journal of Experimental Medicine, the study led by Dr. Andrés Hidalgo’s group at the CNIC shows that neutrophils-a type of white blood cell-have an internal clock that regulates their aggressiveness throughout the day and determines the extent of damage they cause to the heart after a heart attack.