Many joyfully study Torah all night long on Shavu’ot. The reason for this custom is explained in the Zohar: “The early pious ones did not sleep that night, but would busy themselves with Torah…thus when the rabbis gathered at the home of R. Shimon b. Yoḥai on Shavu’ot night, he said: ‘Let us fix the bride’s jewelry so that tomorrow she will be properly adorned before the king.’ Fortunate is the lot of the learned, when the king asks the queen: ‘Who fixed your jewelry and burnished your crown?’ There is no one in the world who knows how to fix the bride’s jewelry other than the scholars; fortunate is their lot in this world and the next” (vol. 3, 98a). Elsewhere the Zohar recounts that R. Shimon and all the rabbis were joyfully studying Torah on Shavu’ot night. Each of them shared original insights, and R. Shimon rejoiced along with them. R. Shimon said to them: “My children, fortunate is your lot. Tomorrow the bride will enter the wedding canopy and only you will accompany her. All those now preparing the bride and sharing her joy will be written in the Book of Remembrance. God will bless them with the 70 blessings and crowns of the supernal world” (vol. 1, 8a).
In order to understand the Zohar, we should clarify that the day of Matan Torah is referred to as a wedding day. It is then that God and the Jewish people formed a special relationship, as do a bride and groom when they get married (Ta’anit 26b). Each year on Shavu’ot, Matan Torah is revisited, and the Jewish people once again renew their relationship with God as if they were bride and groom. According to the kabbalists, studying Torah on Shavu’ot night prepares the Jewish people to receive the Torah in the most delightful way. When the morning arrives, they ascend toward God, deepening and intensifying their connection with Him. As a result, they merit an abundance of Torah, life, and blessing throughout the year.
The holy Arizal said that if one studies Torah all night long on Shavu’ot without sleeping at all, he is guaranteed to live out the year, and is protected from harm all year long, for all of human life hinges on the Torah.
Another explanation is offered for this custom. On the day when the Jews received the Torah, they overslept. Moshe our teacher had to wake them up to receive the Torah, as we read: “Moshe led the people out of the camp toward God, and they took their places at the foot of the mountain” (Shemot 19:17). This was a failure on the part of the people. They neither prepared themselves properly for Matan Torah, nor experienced the appropriate anticipation (Shir Ha-shirim Rabba 1:56). In order to make up for this sin, we study Torah all night on Shavu’ot, as we long for and anticipate the light of Matan Torah, which is revealed anew each year on Shavu’ot (MA §494).
At first, only the pious few would stay up and study Torah all night. Around 400 years ago, thanks to the Arizal’s statement above, the custom to stay up studying all night started to spread and became widely observed. The kabbalists emphatically insisted that those who stay up all night must dedicate the time to intensive Torah study, and not waste time on frivolous matters (Ben Ish Ḥai, Year 1, Bamidbar §3).
In any case, following this custom is not obligatory. If it is difficult for one to stay up all night studying Torah, he may go to sleep. Even some great rabbis preferred to sleep on Shavu’ot night. They evaluated the situation and decided that if they remained awake all night, the loss would outweigh the gain: they would not be able to focus properly on praying in the morning, or they would not be alert enough at night to learn productively, or they would need to catch up on their sleep later on and learn less Torah, or they would not be able to enjoy the rest of the festival properly on account of exhaustion.
On the other hand, those who do stay up all night feel that even if their Torah study is not of the highest caliber, and it is difficult to focus on the morning prayers, this holy custom gives expression to love of God and love of Torah. It has the special advantage of showing dedication to God’s glory. Such dedication enhances the glory of the Jewish people as well. Each person should choose the practice that will allow him to serve God best.