Garmin Watch body battery. Was skeptical initially, but am surprised how well it works. It's a composite of sleep quality for gains (should recharge daily above 80 ideally) and physical stress, measured by heart rate variation, for losses.

Quasi-related to the body battery is minimal heart rate while sleeping (the watches automatic "resting" number is often higher, so I look at the chart). Relative to 2-week moving average it's a leading indicator of illness. I have nice charts for my recent covid and common cold, 1-2 days before the symptoms I could see unexplained (e.g. no drinks) higher levels. In absolute terms it correlates with endurance training and recovery after long training sessions (it goes lower with training in general, after a long session it is higher for 1-3 days, when it's back to low level it's full recovery, good time for further training).

Both help in simple decisions such as go to bed earlier, training intensity, whether evening coding session could be productive or better watch a movie, etc.

As a side note, it's funny how alcohol affects both metrics - same as illness + very low battery the next day. The metrics make me think twice and when both are bad it's better to skip drinking even if the context/schedule and subjective feelings are OK for it.