When I first started on the pipe, quite recently. I bought some Mac Baren HH Old Dark Fired. I only dried out one flake and then smoked it right through. Right after that I felt a complete relaxed sleepy feeling, which I was sure (still am) was down to the tobacco. Went to bed and sleep like a log, woke up the next morning in the fireplace... ha ha sorry. No but it really did help me sleep. I don't really get that now. And also, now I smoke nightcap (which is considered medium/high nic) regularly throughout the day, without any noticeable sleep inducing affect...
I think that:
1) You can build a tolerance to nicotine.
2) Your blood sugar levels and time of day are factors.
Also, I bought some brown rope, a month or so ago... I had read things like "watch out for this", "I couldn't smoke a full bowl" etc.
So I was expecting something powerful and hitting. I smoked it with no real noticeable affect. I didn't get anything except a harsh feeling on the throat. I didn't bother with it again until about 2 days ago, which, this time, kicked my ass a bit. I feel nauseous and light-headed.
So you will get different opinions from different people and even different experiences from the same people, if you catch my drift.
But there are many biological and chemical things going on that, as cosmic says, you can research and also that others on here are a bit more savvy to and can explain.