Some children manage to go through the night without wetting the bed much earlier than others, and evidence suggests that genetics has a major influence over night-time dryness - so if you or your partner got the hang of it easily, odds are your child will do the same (or vice versa).
There is also a physical reason why children reach this stage at different ages.
Within your body is a natural substance called vasopressin that controls the amount of urine your kidneys produce.
At night, the amount of vasopressin increases to slow down the rate at which the bladder fills.
Most children develop the ability to do this between the ages of 3 and 5; some will be dry at night a month after achieving day-time dryness, others may take a year or more to reach this position.
Many children still wear night-time protection at the age of five. Boys generally take longer to reach night-time dryness than girls.
For more information, see Successful Potty Training, published by teach yourself, £6.99