diff --git a/changelog.d/5605.bugfix b/changelog.d/5605.bugfix
new file mode 100644
index 000000000..4995ba9f6
--- /dev/null
+++ b/changelog.d/5605.bugfix
@@ -0,0 +1 @@
+Fix a number of "Starting txn from sentinel context" warnings.
diff --git a/synapse/handlers/account_validity.py b/synapse/handlers/account_validity.py
index 0719da3ab..edb48054a 100644
--- a/synapse/handlers/account_validity.py
+++ b/synapse/handlers/account_validity.py
@@ -22,6 +22,7 @@ from email.mime.text import MIMEText
 from twisted.internet import defer
 
 from synapse.api.errors import StoreError
+from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.types import UserID
 from synapse.util import stringutils
 from synapse.util.logcontext import make_deferred_yieldable
@@ -67,7 +68,14 @@ class AccountValidityHandler(object):
             )
 
             # Check the renewal emails to send and send them every 30min.
-            self.clock.looping_call(self.send_renewal_emails, 30 * 60 * 1000)
+            def send_emails():
+                # run as a background process to make sure that the database transactions
+                # have a logcontext to report to
+                return run_as_background_process(
+                    "send_renewals", self.send_renewal_emails
+                )
+
+            self.clock.looping_call(send_emails, 30 * 60 * 1000)
 
     @defer.inlineCallbacks
     def send_renewal_emails(self):
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index fefba39ea..86f848570 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -253,7 +253,14 @@ class EventsStore(
         )
 
         # Read the extrems every 60 minutes
-        hs.get_clock().looping_call(self._read_forward_extremities, 60 * 60 * 1000)
+        def read_forward_extremities():
+            # run as a background process to make sure that the database transactions
+            # have a logcontext to report to
+            return run_as_background_process(
+                "read_forward_extremities", self._read_forward_extremities
+            )
+
+        hs.get_clock().looping_call(read_forward_extremities, 60 * 60 * 1000)
 
     @defer.inlineCallbacks
     def _read_forward_extremities(self):
diff --git a/synapse/storage/registration.py b/synapse/storage/registration.py
index 983ce1329..13a3d5208 100644
--- a/synapse/storage/registration.py
+++ b/synapse/storage/registration.py
@@ -25,6 +25,7 @@ from twisted.internet import defer
 
 from synapse.api.constants import UserTypes
 from synapse.api.errors import Codes, StoreError, ThreepidValidationError
+from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.storage import background_updates
 from synapse.storage._base import SQLBaseStore
 from synapse.types import UserID
@@ -619,9 +620,15 @@ class RegistrationStore(
         )
 
         # Create a background job for culling expired 3PID validity tokens
-        hs.get_clock().looping_call(
-            self.cull_expired_threepid_validation_tokens, THIRTY_MINUTES_IN_MS
-        )
+        def start_cull():
+            # run as a background process to make sure that the database transactions
+            # have a logcontext to report to
+            return run_as_background_process(
+                "cull_expired_threepid_validation_tokens",
+                self.cull_expired_threepid_validation_tokens,
+            )
+
+        hs.get_clock().looping_call(start_cull, THIRTY_MINUTES_IN_MS)
 
     @defer.inlineCallbacks
     def _backgroud_update_set_deactivated_flag(self, progress, batch_size):
diff --git a/synapse/util/__init__.py b/synapse/util/__init__.py
index dcc747cac..954e32fb2 100644
--- a/synapse/util/__init__.py
+++ b/synapse/util/__init__.py
@@ -62,7 +62,10 @@ class Clock(object):
     def looping_call(self, f, msec):
         """Call a function repeatedly.
 
-         Waits `msec` initially before calling `f` for the first time.
+        Waits `msec` initially before calling `f` for the first time.
+
+        Note that the function will be called with no logcontext, so if it is anything
+        other than trivial, you probably want to wrap it in run_as_background_process.
 
         Args:
             f(function): The function to call repeatedly.
@@ -77,6 +80,9 @@ class Clock(object):
     def call_later(self, delay, callback, *args, **kwargs):
         """Call something later
 
+        Note that the function will be called with no logcontext, so if it is anything
+        other than trivial, you probably want to wrap it in run_as_background_process.
+
         Args:
             delay(float): How long to wait in seconds.
             callback(function): Function to call