This study investigated the effects of fatty fish on sleep , daily functioning and biomarkers such as heart rate variability ( HRV ) , vitamin D status ( serum 25-hydroxyvitamin D ( 25OHD ) , and eicosapentaenoic acid ( EPA , 20:5 n-3 ) + docosahexaenoic acid ( DHA , 22:6 n-3 ) in red blood cells .