The relation of half-life to drug's effects

[psyche]

This is an issue that has been bothering me for some time. Some(many) drugs seem to have a half-life that's way longer than the effects of the drug. Valium, for example has a half-life of 24 hours, yet the effects aren't reported to be more than a few hours. So, what factors affect the duration of subjective effects of the drug so that the drug still stays in your system long long after the effects are supposed to fade away? I have a couple of theories:

This is the weaker one. With nitrous oxide there isn't virtually any metabolism going on, but it quickly redistributes so that the brain doesn't hold much at all N20, and it's excrated almost entirely by lungs. Does the same thing happen with other drugs?

Second one is the brain's normal act of balancing the imbalance. The receptors become more resistive to a drug and down-regulate and what not.

There was an old thread from 2005, asking what is half-life. It only containde couple of one-liners and was old so I thought to make up a thread with more descriptive title.

[psyche]

*bump* Am I missing something obvious? Come on, there must a simple explanation... Confirm/extrapolate on my guesses in the above post if nothing else.