Jane asleep - joke ?

Study of Jane Morris asleep in an upholstered armchair
Rossetti

What I only just read though was Rossetti's barbed joke at Morris 

The book the sitter has been reading is William Morris's first volume of poetry, The Defence of Guenevere, published in 1858. There is a touch of intentional humour in the suggestion that she has found it so soporific. By the late 1860s, when the drawing was probably made, Rossetti was deeply in love with Jane Morris, and her husband was often the victim of his barbed humour. A group of caricatures from this period in which Rossetti openly pokes fun at him is in the British Museum (see J. A. Gere, Pre-Raphaelite Drawings in the British Museum, London, 1994, pp. 46-55, nos. 25-28, 31-32, all illustrated).

According to Virginia Surtees (loc. cit.), the present drawing was formerly inscribed by William Michael Rossetti 'By Dante G. Rossetti of Mrs Willam Morris towards 1870' (on a previous label attached to the backboard).

It was sold at Christie's last year
for £37,250
($58,594)

The question is, did Jane pose for this, knowing the 'joke' ?