The perfect time to be stressed: a differential modulation of human memory by stress applied in the morning or in the afternoon.

We measured the effects of a stressful experience on memory for emotionally arousing and neutral material learned after exposure to a stressor which induces a significant increase in corticosteroid stress hormones. Because memory performance can be influenced by circadian changes in corticosteroid levels, subjects were tested either in the morning or in the afternoon. Nineteen healthy men (9 in the morning group and 10 in the afternoon group) were submitted to a psychological stress task before viewing a story composed of emotionally negative and neutral segments, while another 20 healthy males (10 in the morning group and 10 in the afternoon group) viewed the story without being exposed to the psychological stressor. Salivary cortisol levels were measured before and after the stressor. Memory performance was assessed by a one week post learning delayed recall. Results show that stress-induced increases in salivary cortisol levels impaired delayed free recall of emotionally arousing material in the morning group, but not in the afternoon group. There was no effect of stress on memory for neutral material. Altogether, these findings suggest that stressing participants in the morning, at a time of high circulating levels of corticosteroids, over stimulated the corticosteroid receptors in the brain, impairing declarative memory for emotionally arousing material unrelated to the stressor. These findings suggest that the experimental context, i.e., time of day at which the experiment occurs, the nature of the to-be-remembered material (remembering the stressful event itself or material unrelated to the stressor) and the valence of the to-be-remembered material (emotionally arousing vs. neutral), modulates the effects of stress on human declarative memory.