Direct quote from Elizabeth Pantley's "The No Cry Potty Training Solution" -
"Toilet training has nothing to do with nighttime dryness. Nighttime dryness is achieved only when a child's physiology supports this. A child wets during sleep because of a number of reasons: his kidneys aren't sending a signal to his brain when he's asleep to alert him he has to go, his bladder hasn't grown large enough to contain a full night's supply of urine, his bladder overproduces urine during the night, or he sleeps so deeply he doesn't wake up to go to the bathroom. As children grow, all of these conditions are self-correcting. This usually occurs between the ages of three and six. This isn't something you can teach, and you can't rush it."
"Toilet training has nothing to do with nighttime dryness. Nighttime dryness is achieved only when a child's physiology supports this. A child wets during sleep because of a number of reasons: his kidneys aren't sending a signal to his brain when he's asleep to alert him he has to go, his bladder hasn't grown large enough to contain a full night's supply of urine, his bladder overproduces urine during the night, or he sleeps so deeply he doesn't wake up to go to the bathroom. As children grow, all of these conditions are self-correcting. This usually occurs between the ages of three and six. This isn't something you can teach, and you can't rush it."