Although the custom of most people is to stay awake all night learning Torah and to pray at sunrise, the custom of many Chasidim is different.

According to the Zohar, in addition to the eve of Shavuot, one is to immerse himself in the mikvah an additional time just before dawn on Shavuot morning. The reason stated in the Zohar is to bring upon oneself an extra measure of Holiness (before the "wedding"). At this point in time, when most people are getting ready for Shavuot prayers, the Chasidim go to sleep for a few hours. Most people doze off during the prayers, longing for the end, and missing the most beautiful service of the year. The Chasidim though, awake refreshed after a few hours of sleep and pray with energy and fervor throughout the whole service.

The reason for the delay of the coming of the Mashiach is because the Jews don't draw out the recitation of this prayer….

On Shavuot, the Chassidim have a custom to draw out the saying of the "Ahava Rabba" prayer which comes just before the recitation of the Shema. This custom is in accordance with the Baal Shem Tov who said that it was revealed to him that the reason for the delay of the coming of the Mashiach is because the Jews don't draw out the recitation of this prayer. And if one should pray like this the whole year long, then all the more so on Shavuot!! It is written in the Tikkunei Zohar concerning Shavuot, "On the holiday of Shavuot, we will come out of Exile!"

How beautiful are words said at the appropriate time.

[Based on Zohar and Chassidic sources]