Postgresql Trigger / Transaction Behaviour

Question

To aid me with debugging another issue in my database, I've written the following function as a trigger in postgresql:

CREATE OR REPLACE FUNCTION stage.triggerlogfunction()
 RETURNS trigger
 LANGUAGE plpgsql
AS $function$
begin
    insert into stage.temptriggerlog_complete
    values
    (current_timestamp, 'Hello');
    perform pg_sleep(5);
    insert into stage.temptriggerlog_complete
    values
    (current_timestamp, 'Did');
    perform pg_sleep(5);
    insert into stage.temptriggerlog_complete
    values
    (current_timestamp, 'This Work?');
    return null;
END;
$function$
;

When a new row is inserted into a table in my database, this trigger is set to fire. This works as expected, but what I was expecting to see when this ran was three rows with timestamps that are 5 seconds apart (running in sequence with a 5 second delay between each), but the actual result I had returned was three rows all with the same timestamp.

Why is this happening? Is there a way I can force the behaviour that I was expecting?