Summarising from the Origins thread:

While stanza 1 and 3 appear to be of Burns authorship, the source text for the chorus and stanza 2 likely originated from William Tytler text (ca. 1779) and to a lesser extent David Herd text (ca. 1769).


"Simmer's a pleasant time"
Printed in James Johnson's
Scotts Musical Museum, Vol 3, 1790, p.222

(Burns)
Simmer's a pleasant time,
Flow'rs of ev'ry color;
The water rins o'er the heugh,
And I long for my true lover.

(Tytler)
Ay waukin O,
Waukin still and wearie:
Sleep I can get nane
For thinking on my dearie.

(Tytler)
When I sleep I dream
When I wauk I'm eerie;
Sleep I can get nane
For thinking on my dearie.
Ay waukin &c.

(Burns)
Lanely night comes on,
A' the lave are sleeping;
I think on my bonnie lad
And I bleer my een with greetin'.
Ay waukin &c.



Burns Chorus:
Ay waukin O,
Waukin still and wearie:
Sleep I can get nane
For thinking on my dearie.

Burns Stanza 2:
When I sleep I dream
When I wauk I'm eerie;
Sleep I can get nane
For thinking on my dearie.


William Tytler text:
Ay wa'king oh!
Wa'king ay and wearie;
Sleep I canna get,
For thinking o' my dearie.

When I sleep, I dream;
When I wake, I'm irie:
Rest I canna get,
For thinking o' my dearie.


David Herd text:
O WAT, wat—O wat and weary!
Sleep I can get nane
For thinking on my deary.
A’ the night I wak,
A’ the day I weary,
Sleep I can get nane
For thinking on my dearie.

.