What I’ve learned after 10 years of quantifying myself

I wonder what people's experiences are with this observation, and is there any research out there?

> If you are honest with yourself you have to face the cruel reality: if you are a knowledge worker then on average you can do your job for only 5 hours a day. On bad days it will be 2–3 hours. On the days of super efficiency it would only be 7 — and at the end you will feel totally exhausted. My experience proves that overworking always backfires — the next day after a super day will be much less productive.