Measures of energy in sleep (5)
I believe the answer is:
doses
'measures' is the definition.
(dose is a kind of measure)
'energy in sleep' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'in' indicates putting letters inside.
'sleep' becomes 'doss' (I've seen this before).
'e' inserted inside 'doss' is 'DOSES'.
'of' is the link.
(Other definitions for doses that I've seen before include "Measured portions of medicine" , "Quanitites of medicine" , "Measured amounts of medicine" , "Specific quantities" , "Gives medicine to" .)