Never heard about being susceptible to certain diseases because you sleep during the day instead of the night. Everybody's sleep patterns are determined by their 'internal clocks'. I personally don't get more than 5 hours of sleep each night. But I do take 'power naps' all during the day, and also at night to compensate for the difference. Here's a Wiki on the issue --
Sleep timing is controlled by the circadian clock, sleep-wake homeostasis, and in humans, within certain bounds, willed behaviour. The circadian clock — an inner timekeeping, temperature-fluctuating, enzyme-controlling device — works in tandem with adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine is created over the course of the day; high levels of adenosine lead to sleepiness. In diurnal animals, sleepiness occurs as the circadian element causes the release of the hormone melatonin and a gradual decrease in core body temperature. The timing is affected by one's chronotype. It is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode.[11]
Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep.[12] Along with corresponding messages from the circadian clock, this tells the body it needs to sleep.[13] Sleep offset (awakening) is primarily determined by circadian rhythm. A person who regularly awakens at an early hour will generally not be able to sleep much later than their normal waking time, even if moderately sleep-deprived.
Sleep duration is affected by circadian rhythm which is regulated by a gene named DEC2. Some people have a mutation of this gene; they sleep two hours less than normal. Neurology professor Ying-Hui Fu and her colleagues bred mice that carried the DEC2 mutation and slept less than normal mice