SB 4.23.6

ग्रीष्मे पञ्चतपा वीरो वर्षास्वासारषाण्मुनिः ।
आकण्ठमग्नः शिशिरे उदके स्थण्डिलेशयः ॥६॥

grīṣme pañca-tapā vīro
varṣāsv āsāraṣāṇ muniḥ
ākaṇṭha-magnaḥ śiśire
udake sthaṇḍile-śayaḥ

grīṣme—in the summer season; pañca-tapāḥ—five kinds of heating; vīraḥ—the hero; varṣāsu—in the rainy season; āsāraṣāṭ—being situated within the torrents of rain; muniḥ—like the great sages; ākaṇṭha—up to the neck; magnaḥ—drowned; śiśire—in winter; udake—within water; sthaṇḍile-śayaḥ—lying down on the floor.

Following the principles of forest living and the footsteps of the great sages and munis, Pṛthu Mahārāja accepted five kinds of heating processes during the summer season, exposed himself to torrents of rain in the rainy season and, in the winter, stood in water up to his neck. He also used to simply lie down on the floor to sleep.

These are some of the austerities executed by the jñānīs and yogīs, who cannot accept the process of bhakti-yoga. They must undergo such severe types of austerity in order to become purified from material contamination. pañca-tapāḥ refers to five kinds of heating processes. One is enjoined to sit within a circle of fire, with flames blazing from four sides and the sun blazing directly overhead. This is one kind of pañca-tapāḥ recommended for austerity. Similarly, in the rainy season one is enjoined to expose himself to torrents of rain and in winter to sit in cold water up to the neck. As far as bedding is concerned, the ascetic should be content with simply lying on the floor. The purpose for undergoing such severe austerities is to become a devotee of the Supreme Personality of Godhead, Kṛṣṇa, as explained in the next verse.