Sitting in the sun favors weight loss and this effect is greatest in the early morning hours. The phenomenon was seen with all other factors being the same (calorie intake, physical exercise, body temperature, hours slept, season etc.). Of the many crucial effects attributed to sunlight one is that of controlling the internal clock that regulates circadian rhythms that in turn control energy balance. Thirty minutes exposure between eight o’clock in the morning and midday has visible effects on body weight. Absence of sunlight desynchronizes metabolic processes with consequent increase in weight. It is preferable, but not indispensable, that the light be sunlight. What counts is light intensity. On an overcast day intensity is about 500lux units, a well-lit room gives about 200 lux and a day of sunshine about 1000 lux. MLiT is a new unit that combines three variables: hours of exposure (early morning hours), duration of exposure (at least 30 min), and light intensity (500 lux). The greater MLiT the greater is weight loss. Circadian health is a new concept in medicine and concerns both exposure to light during the day and exposure to darkness at night. Too much light at night (lights on while one sleeps, smart phones and tablets, TV), modifies circadian rhythm and body metabolism through a process of de-regulation of cerebral centers involved in hunger-satiety responses. Surprisingly in children the light- weight relationship is the opposite of what seen in adults. In fact, in children exposure to afternoon light determines more body weight loss than in the morning, whereas similarly to adults, night light helps weight gain. There are about fifty million obese children under the age of five in the world. Among the possible interventions, changes in lifestyle such as having lunch outside the class/school in summer months would contribute to weight loss. Same thing for adults who could have breakfast in the open air where possible.