Autonomic signals changed selectively, not uniformly
Eleven male endurance athletes doubled their usual training volume for two weeks and then completed a one-week taper at half their normal volume. All met the study's overreaching criteria. Parasympathetic control decreased during estimated slow-wave sleep but not across the full four-hour night sample; heart-rate recovery changed after the maximal test but not the constant-speed test. The selected measures returned to baseline after taper.
The study therefore warns against treating one HRV or heart-rate-recovery value as a complete recovery diagnosis. The sample was small, the overload was deliberately severe, and the result depended on measurement window and exercise test. Read those conditions in the PubMed record.
For a supported endurance goal, interpret a repeated morning or nightly signal beside training load, sleep, mood, illness, soreness, and performance at a consistent task. Look for a pattern under comparable measurement conditions rather than reacting to one number. Do not copy the 100% volume increase as a training prescription. Persistent performance loss, unusual fatigue, illness, fainting, chest pain, or other concerning symptoms require recovery and appropriate professional assessment rather than another monitoring metric.