Internal and external purification, contentment, austerity, self-study and devotion to God - are the Niyamas.

Verse 2.31

जातिदेशकालसमयानवच्छिन्नाः सार्वभौमामहाव्रतम् ॥३१॥

jāti-deśa-kāla-samaya-anavacchinnāḥ sārvabhaumā-mahāvratam ॥31॥

Those (the five vows) are not limited by class, place, time and circumstance are extending to all stages constitute the Great Vow.

Verse 2.32

शौच संतोष तपः स्वाध्यायेश्वरप्रणिधानानि नियमाः ॥३२॥

śauca saṁtoṣa tapaḥ svādhyāy-eśvarapraṇidhānāni niyamāḥ ॥32॥

Internal and external purification, contentment, austerity, self-study and devotion to God - are the Niyamas.

The five components of yama are called 'mighty universal vows', as they are not confined to class, place, time or concept of duty. 

They should be followed unconditionally by everyone, and by students of yoga in particular, irrespective of origin and situation, with a reservation concerning cultural

phenomena such as religious ceremonies, vows and vocations of certain people. 

They form the framework of rules on which society is based.

As yama is social practice, niyama evolves from individual practices necessary to build up the sadhaka's own character.

These five observations accord with the five sheaths of man and the elements of nature: earth, water, fire, air and ether. 

As ether (mahat akasa)

is considered as an empty space, outside, so the Atman is an empty space within and is called chit-akasa.

The principles of niyama that are encompassed by kriya-yoga emphasize the importance of self-discipline. Mastery of yoga would be unrealizable without the observance of the ethical principles of yama and niyama.

Cleanliness or purification is of two types, external and internal. Both are necessary. Taking a bath is external purification; performing asanas and pranayama is internal.

Observance of niyama develops friendliness, compassion and indifference, and is a further aid in cleansing the body, mind and intelligence.

PATANJALI YOGA SUTRAS