A tilaka is usually applied for religious or spiritual reasons, or to honour a personage, event, or victory.

What is the importance of tilak?

Tilak, Sanskrit tilaka (“mark”), in Hinduism, a mark, generally made on the forehead, indicating a person's sectarian affiliation. The marks are made by hand or with a metal stamp, using ash from a sacrificial fire, sandalwood paste, turmeric, cow dung, clay, charcoal, or red lead.

Can we sleep with tilak?

Sleep should be done only after one stroke of sunset (about 3 hours). ... It is considered inauspicious to sleep while applying tilak on the frontal.