This is answer to a question from my very sincere Hindu brother, the answer may also help those who wants to seek the truth.
Very good and reasonable question but my explanation became too long. I derived my understanding through my humble study of overall verses of the book Quran.
The reason of abstinence (saum) / traditional known as fasting is to grow in self confidence, self belief and know your self, trust your inner abilities to become a better version of yourself.
Generally mind is a house of lot of uncertainties, emotions, insecurities, apprehensions, desires, hate, anger, jealousy, rivalries, temptations, anxieties, agitation and is full of commotion.
To make this agitated mind calm, sound and at peace it is recommended to observe abstinence popularly known as fasting.
The book Quran says that the purpose of abstinence is to become self aware or to be self conscious (tattaqun) - (2:183).
Abstinence is basically to develop a mindset that we must trust the natural flow in all circumstances. Whatever happens in our life must not effect us. Trust in the wisdom of Nature is imaan in Quranic terminology.
Be humble by knowing that we are very insignificant individual floating through a infinite space in minuscule spaceship called Earth. And we are not it's captain, nothing is in our hand.
What happens if we think too much about our existence or want to take control? Anxieties and desires are born because we give too much importance to ourselves and this keep our mind indulge or busy in unnecessary, useless, idle self talk within known as لَهْوَ الْحَدِيثِ (lahul- hadith) in Quranic terminology hadith are narratives happenings in our mind.
Now comes your main question
1) Abstinence from what?
Abstinence from our urges, desires which are the reason of our anxieties, stress and tensions.
Be self accountable, assess and evaluate yourself, if you lack the ability don't give up, still continue, persist to be on the path of recover and discovery. The best way to educate our self is to start educating those who are in need (miskeen). Educating others is best part of learning. Train your mind to be calm and quite, it is better for you (2:184)
When we quite or calm our mind the revelation takes place, in it is guidance, clarity and criterion that speaks to us (Quran). This notable manifestation (shahr) is called ramadan. Whoever experiences such notable manifestation must practice abstinence - (2:185).
Sorry to share some of my thoughts in between so that you can easily able to relate with your knowledge of epic poem Ramayana. I may be wrong in my assessment but I feel that the word ramadan is a inner journey of cleansing or fighting the evil within just like ramayana in Hindu narration.
The Ramayana is also interpreted as a symbolic representation of the human journey towards self-realization and liberation from the cycle of suffering.
Rama means pleasing, joyful or source of joy and ayanam or ayana signifies journey, travel or path.
The Inner Meaning of Ramayana Rama is the Indweller in every Body. He is the Atma-Rama, the Rama (Source of Bliss) in every individual. His blessings upsurging from that inner Spring can confer Peace and Bliss
Now again coming back to the book Quran,
The guidance is nearer to you than you can realize, you just need to call and you will receive the guidance (2:186).
Now coming to your second question.
2) What is the period which has been misunderstood as dawn to dusk?
Verse 2:187 deals with the solution for our anxieties and our urges. Quran says it is not easy to deal with anxieties and urges, we need to resist, fight and throughly educate our desires / urges until it makes sense to us and clarity is achieved. This is traditionally known as clarity or breakthrough (dawn / fajr) from period of anxiety/ darkness (lail).
For achieving the perfect clarity of thoughts as distinguishable as black and white, one need to use intellectual curiosity and imbibe which is mistranslated as eating and drinking while devoted to the state of humbleness (masjid). In this period one need to restrict oneself from indulging in any type addiction connected with our urges and desires.
These are some steps to bring calmness, stability to mind from agitation.
Fajr is breakthrough thinking mistranslated as dawn,
Lail is anxiety, darkness mistranslated as night.
Kulu is using intellectual curiosity mistranslated as eating
Sharibu is imbibe or absorb the knowledge mistranslated as drinking.
Hopefully I have cleared your doubts.