Fluid builds up during the day.
Here's some info I found
Adrenal Fatigue & Hypoglycemia Syndrome
Although caffeine is commonly used to assist weight loss, with frequent consumption, Haas explains that caffeine's effect of spiking cortisol and adrenaline secretion - hormones secreted by the adrenal glands - can result in adrenal fatigue and hypoglycemia, chronic syndromes in which the body eventually becomes unable to handle stress, resulting in exhaustion, thyroid problems and weight gain.
According to Haas, caffeine is also believed to play a role in the development of fibrocystic breast disease in some women, prostate enlargement in some men, and the incidence of certain cancers including bladder cancer, ovarian cancer and pancreatic cancer.
Caffeine addiction can be a problem, particularly with members of the nine-to-five workforce who use it to help them burn the candle at both ends. Although not as serious as some other drug addictions, withdrawal can be difficult.