a, Pulse transit time (PTT) estimated from the synchronized heartbeat (blue) and wrist pulse (red) waveforms. Inset: one period of the signals and the extracted PTT. b, Probability density distribution of the PTT over 3 min. c,d, Blood pressure extracted from the PTT when the person under test is seated (c) and when the person under test goes through moderate activity and is standing (d). Green markers show blood pressures measured using a commercial blood pressure monitor. Blue and red solid lines are systolic (Sys) and diastolic (Dia) pressures (BP) of every heartbeat. Pink and light blue dashed lines are moving averages from 14 sampling points around 10 s.