Answer

In Jain Dharma, monks and nuns traditionally do not sleep on beds and are expected to practice great simplicity and non-attachment.

Regarding sitting or sleeping on a chair, this depends on the tradition, health condition, and specific monastic discipline.

Explanation

Broadly:

• Digambar monks follow very strict bodily discipline and generally do not use chairs or beds for comfort. They usually rest or sleep on the floor or simple ground space, according to their rules.
• Śvetāmbara Mūrtipūjak monks and nuns traditionally sit and sleep on simple cloth, mat, or wooden plank-like arrangements, not on comfortable beds. Chairs are generally not used for sleeping.
• Sthānakavāsī and Terāpanth monks and nuns also follow simplicity and restraint. They may sit on simple seats or low platforms depending on tradition and discipline, but sleeping in a chair is not a normal practice.

However, in some cases, if a monk or nun is elderly, ill, or medically unable to lie down, a chair-like support may be permitted by their guru or gaccha/tradition as a matter of health necessity, not comfort.

Spiritual Understanding

The purpose of these rules is:

• to reduce attachment to bodily comfort,
• to practice austerity,
• to remain alert in spiritual discipline,
• and to avoid luxury.

So if a monk or nun uses a chair, it should not be understood as ordinary comfort-seeking. It may be due to age, illness, or the specific rules of their sect.

Takeaway

Normally, Jain monks and nuns do not sleep on chairs. If it is seen, it is usually an exception due to health or tradition-specific allowance, not a general Jain monastic practice.