Sahur is a meal that is eaten before early morning prayer in Ramadan.  Sahur is regarded by Islamic traditions as a benefit of the blessings in that it allows the person fasting to avoid the crankiness or the weakness caused by the fast.
The time of sahar ( dawn) is a time when supplications and repentance are accepted. When one gets up for sahur (pre-dawn meal) in Ramadan, one will have gotten up at the time of sahar. We should deem it a great opportunity to make supplication at this time.

In praising good people [in the Qur’an al-karim], Allahu ta’ala says:

“They ask for forgiveness in the last portion of the night” (Adh-Dhariyat 18).

Ya’qub alaihis-salam said to his sons:

“Soon [at the time of sahar], I will ask forgiveness for you from my Lord” (Yusuf 98).

In the 17th verse of Al-i Imran Sura, those who show patience, those who are truthful [in their speeches, deeds, and intentions], those who perform prayer, those who give zakat, and those who ask forgiveness in the last portion of the night are praised. The reason why those who ask forgiveness are mentioned after all others is that humans should consider their every act of worship as defective and should always implore for forgiveness.

Without getting up for Sahur is not sinful, but it is much reward-deserving to get up for pre-dawn meal.

Muhammed (pbh) recommended his companions  that they should begin their fasting by taking something. It does not necessarily have to be a heavy meal. In fact, overeating negates the spirit of fasting. He advised them to wake in the pre-dawn period to appropriately kick-start fasting by eating sahur among other things. It is good to get up for it even if one gets up just to drink a sip of water. It is declared in several hadiths:
Sahur meal is blessed. The whole of sahur is blessing. Get up for sahur even if you drink just a sip of water. Allahu ta’ala and His angels shower salat and salam upon those who get up for sahur.

Eat something for sahur even if it is a date. For there is in it barakah.

Get up for sahur; there is blessing in it.

Aid yourselves in fasting by eating a meal at sahur

Getting up for sahur meal is a blessing that Allah has bestowed upon you. Do not miss it.

Certainly sahur meal is blessed.

Have sahur meal even if it is just a morsel because there is blessing in it. 

How excellent are dates as a Believer’s sahur.

Allahu ta’ala showers mercy upon those who get up for sahur.