Nervous System

• HRV, 7-night average. As in the ladder. In a week with fewer than 3 nights, the newest night stands in (if it is less than 7 days old), and the sheet then calls it "HRV, last night".
• HRV, best 5 minutes. On its own curve centered on 60 ms: 40 ms scores 27, 60 scores 50, 80 scores 73, 100 scores 88. It is then shifted by your own peaks, once you have enough of them.
• HRV against the device's own range. At or above the device's high edge, 95. Between the middle and the high edge, from 70 to 95. Between the low edge and the middle, from 45 to 70. Under the low edge, 45 minus twice the gap, never below 10. It is not drawn on the sheet, but it is in the average.
• Stress. Only the 0 to 100 index (Garmin, and COROS on the same scale) is used here. Minutes of stress from an Oura ring are not, because they have no published scale (stress). Today is left out, because a day still running would move the number through the afternoon. The week's average goes on the stress ladder, and the newest completed day shifts it by up to 20 points.
• Resting heart rate. The week's average on the line 160 minus 1.7 times the rate (50 bpm gives 75, 60 gives 58, 70 gives 41), then shifted by up to 20 points by the newest reading against your own 30-day usual (a rate above your usual lowers the score).
• Lowest heart rate at night. 140 minus 1.8 times the rate (45 bpm gives 59, 50 gives 50, 60 gives 32). No personal shift.
• Body Battery at waking. The device's level at waking, averaged over the last 5 days, taken as it is (0 to 100). It is Garmin's Body Battery, never our Morning Energy estimate.

1. The trend nudge. From your last 7 nights we fit a line through the logarithm of the HRV, read as milliseconds per day at a reference of 40 ms. A rise of 1.5 ms a day or more adds 6 points. A fall of 3.0 ms a day or more takes 5 points. Smaller moves change nothing.
2. Thin data. With only 2 parts the score keeps 60 % of its distance from 50, with 3 parts 80 %, with 4 or more all of it. A score of 80 on two parts is shown as 68.
3. The number is rounded to a whole number.

The index needs at least one part that says something about variability (HRV, the best five minutes, the device's own range, stress or morning Body Battery). A resting rate and a lowest night rate alone are two heart-rate levels and never make a Nervous System. It also needs overnight HRV or a resting rate on at least 3 days, and a reading inside the last 7 days: with none, the card says "No recent data" and shows when it was last measured.