As of August 2010, the National Institute of Health has concluded that studies on the impact of aspirin on blood pressure are largely inconsistent and inconclusive. There is however, a slight increase in the risk of hypertension among aspirin users, cites the NIH.
Taking aspirin at bedtime was shown to reduce blood pressure in a study quoted by WebMD, while taking aspirin in the morning was shown by the same study to raise it. However another study proved an 18 percent increase in blood pressure among aspirin users, quotes the NIH. Due to the inconclusive evidence of aspirin's effects on blood pressure, the NIH recommends that doctors and patients weigh the risks and benefits before beginning an aspirin regimen.