The question is precisely whether they are "really" burnt out or just had a long day (or couple of days) at work and are feeling burnt out at that particular mo

The question is precisely whether they are "really" burnt out or just had a long day (or couple of days) at work and are feeling burnt out at that particular moment. We all experience something akin to burnout at some point or another but it makes a big difference whether it is "I felt burnt out for a day or two after a tough sprint or major production incident" or "I've been working 80h per week for months". The former you might be fine after a relaxing weekend whereas the latter may take months to recover and seriously jeopardize your long-term physical and mental health.