Lord Vishnu is portrayed as Sleeping on a Bedroll of a Huge Snake in Paarkkadal,Ksheera Sagara(Ocean of Milk).
This is allegory.
Lord Vishnu does not sleep.
His sleep is called Yoga Nidra (Sleeping posture in Yoga) or Ari Thuyil(Tamil-sleeping while being aware of everything)
The Snake Adisehsa is the coiled Cosmic energy in the Solar Plexus(Mooladhaara).
The Ksheera Saagara is a representation of the Milky way Galaxy.
Please read my Post Vishnu’s’ Conch and Discus in galaxies, filed under Hinduism/Astrophysics.
This Yoga Nidra of Lord Vishnu is of Nine types.
1.Vadabadra Sayanam, Srivilliputtur.Moolavar Image not available.
2.Veera Sayanam at Thiruindalur near Mayuram or Myiladuthurai.
Image is not available.
3.Bhujanga Sayanam.SrirangamThiruvellore,Thirukkottiyur-in twenty Places
4.Uthara Sayanam, Sarangapani Temple,Kumbakonam
5.Dharpa Sayanam.This is where Lord Rama is said to have meditated lying down to cross over to Sri Lanka .
He sleeps on Grass(Dharpa)
6.Yoga Sayanam.Chidambaram Thiruchithra Koodam
7.Manikka Sayanam. Thiruneermalai, near Chennai. Moolavar Image not available.
8.Bhujanga Sayanam. SriRangam.
9…Ananatha Sayanam. Thiruvananthapuram,Kerala.
I shall be posting on the significance of each posture