I'm aiming to reduce my caffeine intake, but don't want to drop coffee altogether. What roasts / beans release the least amount of caffeine into my body when drank?
In general, darker, arabica roasts have less caffeine. Arabica (as opposed to robusta) inherently has less caffeine in it naturally, and darker coffees have less caffeine since they have been roasted longer or at higher temperatures,
essentially "burning" the caffeine out of the bean.
For some science, Arabica is usually found to be about 1.2% caffeine and Robusta is typically 2.2% caffeine
Chris in AK has a great explanation in the comments as to why darker roasts are perceived to have less caffeine
It is a common misconception that darker roasts have less caffeine. Caffeine's melting point is 455F which is well above what most beans reach during the roasting process. Essentially, caffeine content changes because the mass/density of the beans change (they lose water and undergo changes in the roasting process). If you measure your coffee by volume, dense greener beans will have more caffeine. If you measure by weight, darker roasted beans will have more caffeine.
Caffeine is chemically stable even when passed through its boiling point (evaporation temperature) of 312°C (593.6°F), as demonstrated in this study, Thermoanalytical study of purine derivatives compounds by Schnitzler et al (Ecl. Quím., São Paulo, 29(1): 71-78, 2004):
The evaporation of caffeine occurs without decomposition, in compliance with the infrared spectra of caffeine and solidified sample collected after evaporation...(page 75)
Even a dark roast cannot proceed for long past the "second crack" temperature:
When the coffee reaches approximately 224 °C (435 °F), it emits a "second crack", this sound represents the structure of the coffee starting to collapse. If the roast is allowed to progress further, the coffee will soon fully carbonize, and eventually combust.
While the caffeine is not significantly affected by roasting, much else about the coffee beans is transformed (increased volume, loss of moisture, carmelization of sugars), thus changing the mass and density of the coffee bean.
If your goal is to reduce caffeine intake you don't necessarily have to change the type of coffee you're drinking - only the quantity.
I too felt the need to cut down on my caffeine intake. I solved it by letting myself drink all the coffee I want until lunch time and then switch to tea in the afternoon and night.
This way I get to keep my caffeine consumption down and still drink the coffee I want. As far as I know (and see Kyle's and hardmath's posts) the type of roast doesn't appreciably affect the caffeine level/