diff --git a/synapse/storage/event_push_actions.py b/synapse/storage/event_push_actions.py
index 1b7888a914ee..808c9d22fc2e 100644
--- a/synapse/storage/event_push_actions.py
+++ b/synapse/storage/event_push_actions.py
@@ -26,6 +26,42 @@
 logger = logging.getLogger(__name__)
 
 
+DEFAULT_NOTIF_ACTION = ["notify", {"set_tweak": "highlight", "value": False}]
+DEFAULT_HIGHLIGHT_ACTION = [
+    "notify", {"set_tweak": "sound", "value": "default"}, {"set_tweak": "highlight"}
+]
+
+
+def _serialize_action(actions, is_highlight):
+    """Custom serializer for actions. This allows us to "compress" common actions.
+
+    We use the fact that most users have the same actions for notifs (and for
+    highlights).
+    We store these default actions as the empty string rather than the full JSON.
+    Since the empty string isn't valid JSON there is no risk of this clashing with
+    any real JSON actions
+    """
+    if is_highlight:
+        if actions == DEFAULT_HIGHLIGHT_ACTION:
+            return ""  # We use empty string as the column is non-NULL
+    else:
+        if actions == DEFAULT_NOTIF_ACTION:
+            return ""
+    return json.dumps(actions)
+
+
+def _deserialize_action(actions, is_highlight):
+    """Custom deserializer for actions. This allows us to "compress" common actions
+    """
+    if actions:
+        return json.loads(actions)
+
+    if is_highlight:
+        return DEFAULT_HIGHLIGHT_ACTION
+    else:
+        return DEFAULT_NOTIF_ACTION
+
+
 class EventPushActionsStore(SQLBaseStore):
     EPA_HIGHLIGHT_INDEX = "epa_highlight_index"
 
@@ -58,15 +94,17 @@ def _set_push_actions_for_event_and_users_txn(self, txn, event, tuples):
         """
         values = []
         for uid, actions in tuples:
+            is_highlight = 1 if _action_has_highlight(actions) else 0
+
             values.append({
                 'room_id': event.room_id,
                 'event_id': event.event_id,
                 'user_id': uid,
-                'actions': json.dumps(actions),
+                'actions': _serialize_action(actions, is_highlight),
                 'stream_ordering': event.internal_metadata.stream_ordering,
                 'topological_ordering': event.depth,
                 'notif': 1,
-                'highlight': 1 if _action_has_highlight(actions) else 0,
+                'highlight': is_highlight,
             })
 
         for uid, __ in tuples:
@@ -202,7 +240,8 @@ def get_after_receipt(txn):
             # find rooms that have a read receipt in them and return the next
             # push actions
             sql = (
-                "SELECT ep.event_id, ep.room_id, ep.stream_ordering, ep.actions"
+                "SELECT ep.event_id, ep.room_id, ep.stream_ordering, ep.actions,"
+                "   ep.highlight "
                 " FROM ("
                 "   SELECT room_id,"
                 "       MAX(topological_ordering) as topological_ordering,"
@@ -243,7 +282,7 @@ def get_after_receipt(txn):
         def get_no_receipt(txn):
             sql = (
                 "SELECT ep.event_id, ep.room_id, ep.stream_ordering, ep.actions,"
-                " e.received_ts"
+                "   ep.highlight "
                 " FROM event_push_actions AS ep"
                 " INNER JOIN events AS e USING (room_id, event_id)"
                 " WHERE"
@@ -272,7 +311,7 @@ def get_no_receipt(txn):
                 "event_id": row[0],
                 "room_id": row[1],
                 "stream_ordering": row[2],
-                "actions": json.loads(row[3]),
+                "actions": _deserialize_action(row[3], row[4]),
             } for row in after_read_receipt + no_read_receipt
         ]
 
@@ -311,7 +350,7 @@ def get_unread_push_actions_for_user_in_range_for_email(
         def get_after_receipt(txn):
             sql = (
                 "SELECT ep.event_id, ep.room_id, ep.stream_ordering, ep.actions,"
-                "  e.received_ts"
+                "  ep.highlight, e.received_ts"
                 " FROM ("
                 "   SELECT room_id,"
                 "       MAX(topological_ordering) as topological_ordering,"
@@ -353,7 +392,7 @@ def get_after_receipt(txn):
         def get_no_receipt(txn):
             sql = (
                 "SELECT ep.event_id, ep.room_id, ep.stream_ordering, ep.actions,"
-                " e.received_ts"
+                "   ep.highlight, e.received_ts"
                 " FROM event_push_actions AS ep"
                 " INNER JOIN events AS e USING (room_id, event_id)"
                 " WHERE"
@@ -383,8 +422,8 @@ def get_no_receipt(txn):
                 "event_id": row[0],
                 "room_id": row[1],
                 "stream_ordering": row[2],
-                "actions": json.loads(row[3]),
-                "received_ts": row[4],
+                "actions": _deserialize_action(row[3], row[4]),
+                "received_ts": row[5],
             } for row in after_read_receipt + no_read_receipt
         ]
 
@@ -418,7 +457,7 @@ def f(txn):
             sql = (
                 "SELECT epa.event_id, epa.room_id,"
                 " epa.stream_ordering, epa.topological_ordering,"
-                " epa.actions, epa.profile_tag, e.received_ts"
+                " epa.actions, epa.highlight, epa.profile_tag, e.received_ts"
                 " FROM event_push_actions epa, events e"
                 " WHERE epa.event_id = e.event_id"
                 " AND epa.user_id = ? %s"
@@ -433,7 +472,7 @@ def f(txn):
             "get_push_actions_for_user", f
         )
         for pa in push_actions:
-            pa["actions"] = json.loads(pa["actions"])
+            pa["actions"] = _deserialize_action(pa["actions"], pa["highlight"])
         defer.returnValue(push_actions)
 
     @defer.inlineCallbacks