The Sikh Masters taught the value of ishnaan – or hydrotherapy – as part of the daily practice of Sikh Dharma. Traditionally, in India, ishnaan is done by dipping in the sacred pools that surround the Sikh temples. In modern times, a cold shower in your own home can provide the same benefits.
Sikhs believe that when you take a cold shower in the morning, it is like the first battle of the day. Shouting “Wahe Guru!” when you are shivering, is your battle cry; and when you come out victorious you are ready to face any challenge, come what may.
The Gurus set the example, beginning with Guru Nanak, who used to meditate in the river early in the morning. The practice is called ishnan, and is the science of hydrotherapy. It totally revitalizes your capillaries and makes you bright, fresh, and alert for your sadhana (the next topic shares more about Sadhana).
If during your shower you recite the Shabad (Sacred Song) by Guru Ram Das, Gur Satigur Ka Jo Sikh Akhaae then the combination creates a vibration and experience similar to taking a cold dip in the Amrit Sarovar (sacred pool) at the Golden Temple in Amritsar.
Resource: Living Reality (1994) by Bibiji Inderjit Kaur Khalsa.