In a daring experiment, researchers at the Brigham Women's Hospital found that just 21 days of irregular sleeping hours is enough to turn healthy participants into pre-diabetics. 
Twenty-one apparently healthy participants, of both genders and aged in their 20s and 60s, lived in a controlled environment for six weeks. Researchers monitored and adjusted the food, sleep times, and activities. For each of the first 12 days, the volunteers slept for around 10 hours, which is a healthy amount of sleep.
The amount of sleep time was then reduced to 5.6 hours per 24-hour day for the next three weeks, with the sleep starting at different times. This was done to simulate a person living a busy life, and also the effects of rotating shift work. In effect, the researchers simulated a 28-hour day to shift the bio-rhythms (circadian rhythms) and observe the effect on the volunteers' glucose levels and metabolism. For the last 9 days of the experiment, all of the participants were allowed to adjust back to sleeping normally for 10 hours a night.
The reduced sleep, and shifted sleeping times led to the volunteers' bodies making more glucose after each meal and 32% less insulin. Together, these observations were indicative of some of the participants becoming pre-diabetic. Researchers also noticed an eight percent reduction in the participants' metabolic rate, which equates to a weight gain of 12.5Ib per year. During the final 9 days of the study, the changes to the participants' insulin, glucose, and metabolic rate reversed back to healthier levels.
Concerns about the study
In some ways, the research failed to closely replicate real life. The volunteers saw no clocks, and always lived in a dimly-lit environment, as if they resided on a space station. This is not how people live every day; we see daylight, experience sunlight, and breathe fresh air, even if in some cases it is just for a few minutes. In addition, the imposition of a '28-hour day' has little real world relevance. Some of the participants lost a little of their body weight (1%), although whether the loss was of muscle or fat is unknown, and the researchers could say what caused it.
Real world implications (for people who don't live in a lab!)
When you consistently get to bed at irregular, late times, your body's blood glucose and insulin levels are affected, and you move towards being diabetic. This slowly leads to weight gain and obesity. In simple language, sleeping late gains weight.
I have seen slim women in their twenties gain 200Ibs in just five years by living a stressful life of staying up late consistently, working long hours, and failing to exercise and eat healthily. The research describes what happens in the real world.
Just three weeks of inactivity and irregular sleep can dramatically change how the human body functions. For some people, staying up late for 21 days happens when they have a busy workload at their job or at university. For others, the small hours of the night is the only quiet time they get, and so they watch late night television. Some people are required to be alert because they choose to work for the military, security companies, or emergency services.
Whatever your reason, being awake at night for a prolongued number of weeks or months, or getting insufficient night time sleep is harmful to your health. Between 10pm and 2am is the time period when the human body does most of its repair work. If you are up during that time, your body is unable to repair itself fully, and eventually, parts of it fail to work properly, leading to diabetes and other diseases.
We are daytime creatures, and have been for millenia. As a part of nature we are meant to be resting and asleep during the hours of darkness, like many other mammals. Trying to fight nature by sleeping less, sleeping irregularly, and working shifts damages the human body. Nature fights back by making our bodies more likely to produce a stroke or heart attack, and cancer when we consistently sleep late and work shifts.
Research has also shown that people who get to bed around 10 or 10:30 lose more weight than people who stay up late. The clients I see who go to bed at a reponsible, healthy time (10 - 10:30pm) look younger, are slimmer, and are successful in many areas of their lives; they know that to get the most out of their body, they have to take care of it.
Infrequent, short periods of sleep disruption can be handled by a healthy body, which recovers after the person reverts to their healthy sleeping habits. When we are in poor health, the effects of sleep disruption and shifts can be more damaging, and takes longer to recover from; an unhealthy body is less able to cope with the stress of sleep disturbances and needs longer to heal.
 Buxton, O. M. et al. (2012). Adverse metabolic consequences in humans of prolonged sleep restriction combined with circadian disruption. Science Translational Medicine. Pub. April 11, 2012.
 Chek, P. (2002). Nutrition and lifestyle seminar, level 1. CHEK Institute. Brighton, UK. September, 2002.