心率变异性（HRV）正成为智能手表中最有价值的健康指标之一。它衡量的不是心跳快慢，而是心跳间隔的变化程度；与心率不同，HRV通常“越高越好”。其生理基础在于自主神经系统（ANS）的双重调节：心脏的内在节律约为每分钟100次，交感神经（“战或逃”）通过激素和递质使心跳更均匀、HRV降低；副交感神经（“休息与消化”）经迷走神经进行精细的电信号调控，使心跳间隔更不规则、HRV升高。

各种压力都会降低HRV，无论是身体还是心理。高强度训练、睡眠不足、感染、情绪困扰或财务压力，都会使HRV在数小时甚至数天内下降。群体层面上，较高的HRV意味着更具韧性的自主神经系统，与更低的心梗风险、更高的心梗生存率相关（该指标最早在心内科病房证明价值），还与更慢的痴呆进展、更低的炎症水平和更低的抑郁风险相关。

对个人而言，HRV可视为“累积压力表”。运动者的低HRV可能提示训练负荷过大，需要恢复；但解读必须结合情境：一次长骑行、饮酒（酒精抑制HRV）或突发压力都会让次日HRV走低。因此，结合HRV记录生活事件的日志，才能把数据转化为可行动的信息，而非误读的数字。

Heart-rate variability (HRV) is emerging as one of the most useful metrics on smartwatches. It measures not how fast the heart beats, but how variable the intervals are; unlike heart rate, higher HRV is generally better. The physiology reflects dual control by the autonomic nervous system (ANS): the heart’s intrinsic rhythm is about 100 beats per minute, while the sympathetic “fight-or-flight” arm smooths intervals and lowers HRV, and the parasympathetic “rest-and-digest” arm—via the vagus nerve—adds fine-grained electrical control that raises HRV.

All kinds of stress reduce HRV, physical and psychological alike. Hard training, poor sleep, infection, emotional strain or financial worry can depress HRV for hours or days. At the population level, higher HRV signals a more adaptable ANS and is linked to lower heart-attack risk and higher survival if one occurs (where HRV first proved its worth), as well as slower dementia progression, less inflammation and lower depression risk.

For individuals, HRV works best as an “accumulated stress” gauge. For athletes, a low reading may warn of excessive training and the need to ease off—but context matters. A long ride, alcohol (which suppresses HRV) or an unexpected stressor can all lower next-morning values. Pairing HRV charts with a simple journal helps turn raw data into useful information.