My wife is experiencing extreme tiredness. still tired after a full night's sleep as tracked by smart watch. gets tired during the day and even falls asleep throughout the day sometimes, even after consuming 4 cups of coffee. for months now. thanks?

A sleep disorder is one possible cause. Avoid fluids and eating , electronic devices,two hours or less before bedtime, and keep sleep environment clean and quiet. If not improved, she should consult her physician. A virtual consultation with HT is available.

Agree that the most likely answer is a "sleep disorder", and would definitely consider Narcolepsy. But would also check hematological function for anemia, and exclude hypothyroidism. See neurologist or sleep specialist.