I went through the answers and I am utterly disappointed with two things-
1) The answer focus more on providing solution (cheeky answers written for upvotes) rather than understanding the nature of the question
2) Not showing empathy towards the asker (asker seems more keen on knowing why he has underwent such a change and is curious about this unexplained phenomenon)

Before I answer the question I want to assure the asker that this is perfectly normal and that right now that makes two of us.

And now the answer-

The difference between the phenomenon that asker is suffering from and that I am yet experiencing (I don't consider this this as suffering and I am perfectly ok with late to bed and late to rise concept) is that asker has experienced the change two years ago while I have had such kind of 'patches' throughout my life. Having access to genetics and animal development (my favorite subjects during graduation) and my casual interest in neuroscience helped me to understand this problem to a little deeper extent. Its was not a 'research project - like' approach but a thirst to quench my burning curiosity. And so one day, I finally stumbled upon this thing called the Biological Clock .

Our body responds to the cycles of nature. We all know that seasonal changes affect the surrounding. It affects the crops, birds, animals, insect and even us humans. But do you know that there is another cycle that affect us daily and embeds a pattern in our body? It is the Circadian cycle or simply the day and night cycle, the real culprit behind the whole problem. The Circadian clock in our body responds to this cycle and brings about changes in our body those affect our daily habits. Circadian clock is a biological clock.  A biological clock is defined as-

An innate mechanism that controls the physiological activities of an organism which change on a daily, seasonal, yearly, or other regular cycle.

Circadian clock is a biological clock that drives the circadian rhythms in our body. Circadian rhythms are oscillations of biological activity based on a 24-hour period and influenced by regular variations in the environment, such as the alternation of night and day.

Sleep is regulated by two body systems:  sleep/wake homeostasis and the circadian biological clock. Sleep/wake homeostasis tells us that a need for sleep is accumulating and that it is time to sleep. Circadian biological clocks regulate the timing of periods of sleepiness and wakefulness throughout the day.
(Full article: Sleep Drive and Your Body Clock)

Did You Know? Why  do  you  sometimes  wake  up  on  time  even  if  your  alarm  clock  doesn't  ring?  How  do  nocturnal  animals  know  when  it  is  time  to  wake  up?  It's  because  you—and  most  other  animals—have  a  kind  of  internal  clock  that  controls  the  cycle  of  the  day's  biological  activities,  such  as  sleeping  and  waking.  These  daily  biological  activities  are  known  as circadian rhythms because  they  are  influenced  by  the  regular  intervals  of  light  and  dark  in  each  24-hour  day.  While  the  process  underlying  circadian  rhythm  is  not  completely  understood,  it  is  mainly  controlled  by  the  release  of  hormones.  The  brain  regulates  the  amount  of  hormone  released  in  response  to  the  information  it  gets  from  light-sensitive  cells  in  the  eye,  called photoreceptors. Circadian  rhythms  can  be  disrupted  by  changes  in  this  daily  schedule.  For  example,  biologists  have  observed  that  birds  exposed  to  artificial  light  for  a  long  time  sometimes  build  nests  in  the  fall  instead  of  the  spring.  In  humans  who  travel  long  distances  by  air,  the  local  time  of  day  no  longer  matches  the  body's  internal  clock,  causing  a  condition  known  as jet lag.

Source- http://www.thefreedictionary.com...

So, it is the disturbance in our circadian biological clocks that causes weird sleeping patterns. Although, usually the disturbances occur due to change of habits, they may also be inborn either - inherited or result of genetic mutations/recombination.

Here are some interesting research articles:
Gene Mutation Upsets Mammalian Biological Clock
Scientists discover a protein that silences the biological clock
Natural selection against a circadian clock gene mutation in mice

So, one may find that his response to circadian cycle of nature is very different than from others. While others feel active at day, he/she may feel more active at night. And this may have nothing to do with your habits and discipline.

NOTE (update): The question seems to have changed after I started writing answer. The answer is written w.r.t. original question and makes more sense that way.