I totally agree with couldbeworse.
High blood pressure can and does have this effect on the small vessels in the eyes. They simply burst. With the reduction of blood pressure, (= if the cause is treated) they repair themselves. Red eyes can have several causes, high blood pressure is one of them. I have hypertensive retinopathy. I had woken several times over the past few years with bloody eyes. I wish I had seen the doctor then, instead of blaming the redness on spider bites, allergic reactions, having too much to drink, or whatever else I could think of. I urge anyone with red eyes for no apparent reason to see an optometrist or an opthalmologist, or a GP at the very least.
PS. I am sure I messed up the spelling of the opt. doctor.