Why person tends to sleep, when leaning.

Many times you would have self observed that sitting and reading in the way shown in the image, most of the times finally ends up 
you to sleep after around 15 to 20 minutes.

Yes,

you read it right we all start to feel drowsy after sometime while sitting in this position. 

So, this is the way i was talking about. You see the spine region getting subjected to pressure. 

It is Thoracic vertebrae, which is being subjected to pressure.

 Image source: pilates.about.com

 Thoracic vertebrae, are numbered from T1 to T12. So, just remember that whenever you feel the way i said; sleepy, drowsy kind of think to straiten your spine.

By not leaning to study, read, etc.

Else you know you are ultimately going to sleep.

Let me share what wikipedia explains in brief about Thoracic Vertebrae.

"In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae.^[1] In humans, there are twelve thoracic vertebrae and they are intermediate in size between the cervical and lumbar vertebrae; they increase in size going towards the lumbar vertebrae, with the lower ones being a lot larger than the upper. They are distinguished by the presence of facets on the sides of the bodies for articulation with the heads of the ribs, and facets on the transverse processes of all, except the eleventh and twelfth, for articulation with the tubercles of the ribs. By convention, the human thoracic vertebrae are numbered T1-T12, with the first one (T1) located closest to the skull and the others going down the spine towards the lumbar region."

- en.wikipedia.org