What does a consistent sleep schedule do to your heart rate?
A consistent sleep schedule protects the nervous system's control of heart rate. The strongest effects on heart rate become visible when you also align your mealtimes with your sleep pattern: that measurably lowers your nighttime heart rate.
People who go to bed and wake up at a different time every day throw the nervous system that controls heart rate out of balance. Many studies combined, covering 59 studies in total, show that irregular sleep times go hand in hand with an imbalanced nervous system, higher blood pressure, and a greater chance of heart and blood vessel disease. Cause and effect have not been firmly proven, but the link is consistent and of moderate certainty, even by the researchers' own assessment.
A consistent sleep schedule on its own seems to have little direct short-term effect on resting heart rate. A small study in which people followed a strict schedule for 12 nights showed that their sleep did indeed become more regular, but blood pressure and circulation did not change. In other words, a few weeks of greater regularity is probably not enough to produce measurable physical changes.
What did show a clear effect was planning the last meal at least three hours before bedtime, timed to fit your own sleep pattern. After around seven to eight weeks, people with excess weight had a lower nighttime heart rate and better heart rate balance than the comparison group. Heart rate balance is the tiny fluctuation between consecutive heartbeats. The larger that fluctuation, the better your nervous system maintains the balance between rest and alertness, so this is a good sign.
Too little sleep disrupts heart rate balance, and that is considered a sign that the body is under strain. The same pattern appears in people who regularly work night shifts. Light therapy did not help in that group, but the approach was only tried for four weeks.
This answer draws on a large overview of 59 studies, two separate reviews of the existing literature, and three smaller studies involving a total of tens of participants. The researchers behind the large overview rate the evidence as consistent but of moderate certainty: irregular sleep times are linked to higher blood pressure, difficulty processing blood sugar, and heart and blood vessel problems, but studies in people that definitively separate cause and effect are largely absent. The smaller studies offer early indications of what a consistent schedule or an adjusted eating pattern can do, but are too small to support firm conclusions.