Natyashastra
एकं भुजमुपाधाय सम्प्रसारितजानुकम् । स्थानं प्रसारितं नाम सुखसुप्तस्य कारयेत् ॥ २३१॥
Resting the head on one arm and stretching the knees — this posture is called Prasārita and should be used to depict someone enjoying a pleasant sleep.
english translation
ekaM bhujamupAdhAya samprasAritajAnukam । sthAnaM prasAritaM nAma sukhasuptasya kArayet ॥ 231॥
hk transliteration by Sanscript