Heart specialists know that many heart attacks and strokes happen in the early hours around dawn - and this week scientists have worked out why we may be at our most vulnerable at that time.
Cells that line our blood vessels are at their most inactive at around dawn, and, as a result, arteries become more rigid and less able to repair themselves.
The activity of these cells, known as endothelial progenitor cells (EPCs), seems to be influenced by the daily, or circadian, rhythms in the brain. EPCs are at their most active, and numerous, at around midnight, and then decline dramatically until the dawn.
Researchers from Emory University in the USA discovered the activity patterns of EPCs when they monitored 12 volunteers every four hours over a 24-hour period.
(Source: BBC website, November 11, 2008).