Coffee anyone?

You might want to take a good multi vit with minerals, if you're not feeling very perky! Your body may be wanting more B complex (along with others), hence the feeling of needing more caffine

Well, yes and no. Your metabolism does change just about everything, but as you get healthier that usually moves in the opposite direction (more energy). If you're still actively losing weight you may be feeling your calorie deficit - you're running on empty and your body knows it, caffeine is a natural stimulant so it chemically makes up for that void but it can only do so much. Your body is slow to adapt to your new eating habits and you're likely to go through all kinds of swings in the meantime.

Really, though, it depends on what you mean by "function." Here's the basic idea of how caffeine works, without the crazy complicated bits: you have a chemical in your brain called adenosine, its job is to shut down parts of your brain that have been working too hard or too long, this is the chemical that makes you feel tired and which makes your thinking get fuzzier the closer to nap time you get. Caffeine has the ability to bond to the receptor sites for adenosine in your brain, but without actually turning anything off, thus stopping it from shutting down your neurons. This is why you crash so hard after a caffeine buzz, by the way, while the caffeine is sitting on those sites, huge amounts of adenosine are building up in your brain - when the caffeine finally falls off they all bond at once.

So from the above we know that caffeine itself doesn't actually do squat for energy, so why the buzz? Well when your body's control mechanisms don't work - like when adenosine can't shut your brain down - it panics. Your body takes its inability to turn itself off as an emergency state and dumps adrenaline, among other chemicals, into your bloodstream. This is where the energy boost actually comes from. Now it may be that your increased level of fitness and activity has changed the way your body responds to stress. Working out is a kind of stress, and if you're doing it often you may be teaching your body a different "scale" of emergency states which may result in your morning coffee releasing less of those energy-producing chemicals.

Either way, keep in mind that your body is in a state of flux, you are transitioning between one weight/lifestyle and another. Coffee will start working better again, especially if you take a short caffeine detox and let your body un-learn its responses to that particular chemical.