Significant changes covering court's antisocial work periods (9,6)

'antisocial work periods' is the definition.
(late-night work shifts)

'significant changes covering court's' is the wordplay.
'significant' becomes 'grave' (grave can mean significant or urgent).
'changes' becomes 'shifts' (shift can mean a change).
'covering' indicates putting letters inside.
'court' becomes 'yard' (courtyard).
'grave'+'shifts'='graveshifts'
'graveshifts' going around 'yard' is 'GRAVEYARD SHIFTS'.