Halliday & Matthiessen (1999: 126):
In addition, one or more of the figures in the [sequence] may be realised by something ‘less than’ a clause. One major source of this is ideational grammatical metaphor […]. But there are also other cases such as circumstances of Rôle with temporal implications — for example: as a child, he was very shy ‘when he was a child, he was very shy’.