This is one of those questions where the published answer is stronger than most people expect — and more specific.
What did the study actually find?
In 2024, researchers published a study in Frontiers in Digital Health that read the cardiac signal from an ordinary wrist wearable — once per second, through normal daily life, no lab and no prompts — and asked whether that signal could identify how people said their day had felt. It could: the model identified self-reported low-mood days with 90% accuracy.
The same paper carries a quieter finding that deserves more attention: the signal was much worse at predicting energy — how much people felt they had in the tank. Mood, it read well. Fuel level, it did not.
Why would a heart signal know anything about mood?
Because the heart is downstream of the brain’s valuation machinery. Dopamine and oxytocin — attention and emotional resonance — act on the heart through the nervous system, and their effects leave a signature in the heart’s rhythm. A day whose moments held you and moved you writes a different rhythm than a day that merely passed through you. Read that signature continuously and the shape of the day is in the signal before you have put words to it.
That is also why the energy finding makes sense. The rhythm carries what your day meant better than what it cost — meaning and expenditure are different quantities, and your strap already has the cost side covered.
What does this mean for the strap you already wear?
Your strap sends a cardiac signal at the same once-per-second cadence the study used. What the research adds is that this stream carries more than effort and recovery: it carries how the day landed.
The step that makes it useful is context. A prediction that “yesterday was a low day” arrives too late to act on; a reading attached to the hour it came from — this meeting, that workout, the dinner after — tells you what to do more of. That is the design behind brainful — the b-score app for Whoop straps: it reads the signal your strap already sends and puts a b-score (0–100) on each calendar event, a measure of what each hour was actually worth, while the day is still happening.
Source: Merritt, S. H. & Zak, P. J. (2024), “Continuous remote monitoring of neurophysiologic Immersion accurately predicts mood,” Frontiers in Digital Health. The study frames its findings as screening research; brainful is not a medical product, makes no screening claims, and nothing here is medical advice.