According to the Harvard Medical School, as quoted by Parenting Squad, it takes approximately one week to adjust to a new sleeping schedule. That time can be shortened by changing one's eating habits, avoiding caffeine and adjusting light levels.
A study from the Harvard Medical School recommends not eating for 12 to 16 hours before the desired wake-up time when adjusting to a new sleeping schedule. The body's "food clock" will override the natural circadian rhythms, which are driven by light. At the new wake-up time, the body will decide that it must be breakfast time since it is so hungry and will reset to the new schedule.
Other suggested tricks for adjusting to a new sleep schedule include avoiding caffeine for 12 hours before bedtime and darkening rooms early to get the body ready for sleep.