For Daily weighers....

Losing 1-1.5% of body weight (fluid loss) during exercise is fairly normal. One has to keep an eye on their fluid intake before, during, and after exercise. Weighing before and after is a simple way to regulate how much water you have lost. Waking up the morning weighing less than before you went to bed usually is related to water loss during the night (sweat, urination, using the toilet). Again - it's very normal.

Symptoms by percent body weight water loss:

PERCENT WATER LOST

---

SYMPTOMS
0% --- none, optimal performance, normal heat regulation
1% --- thirst stimulated, heat regulation during exercise altered, performance declines
2% --- further decrease in heat regulation, hinders performance, increased thirst
3% --- more of the same (worsening performance)
4% --- exercise performance cut by 20 - 30%
5% --- headache, irritability, "spaced-out" feeling, fatigue
6% --- weakness, severe loss of thermoregulation
7% --- collapse likely unless exercise stops
10% -- comatose
11% -- death likely

[Nutrition for Cyclists, Grandjean & Ruud, Clinics in Sports Med. Vol 13(1);235-246. Jan 1994]

I find the fluctuations fun! I can gain as much as 3 lbs. in a day. I really get a kick out of weighing myself right before and after a bowel movement. It actually keeps me sane. So I know that I am only fluctuating. I wont count a loss/gain unless it sticks throughout the day for 5 days or more.