I saw in some books (Rav Belsky, Arizal) that you are not allowed to learn torah Shebictav at night without mefarshim. Why is that?
In the siddur of Rabbi Shabtai from Rashkov, which presents kavvanot of the Arizal (many books do, it's just the one I have at hand), it says, in the section 'Kriat Shma she-al Ha-Mita':
For the Kabbalists, part of the rhythm of time is that during the first half of the night the forces of judgment are ascendant in the world of עשיה. Also, for the Kabbalists, different kinds of learning--e.g., learning Mishnah, learning Kabbalah, learning Miqra-- stand in different worlds. Since Miqra stands in עשיה, it shouldn't be engaged in at night.