I've been noticing this for a while, but it was confirmed recently. I had to stay up to study, but the coffee and energy drinks I consumed didn't keep me awake. I thought "I just have really high tolerance now, but if I drink a ****load of it it will still take effect". So I bought a a few red bulls and drank them while I studied. I still got very tired, to the point where studying was useless, but when I went to bed I couldn't sleep at all. A couple of years ago a 2 or 3 red bulls would have me energetic to the point where someone might mistake me for a tweaker, but now all the positive effects are gone and the negative ones are amplified.