diff --git a/runfixnights.py b/runfixnights.py
index cd2ea2c..33cc05d 100644
--- a/runfixnights.py
+++ b/runfixnights.py
@@ -74,6 +74,7 @@ Rota.constraint_options["prevent_thursdays_before_full_weekends"] = [
     "exeter",
     "truro no proc",
 ]
+Rota.constraint_options["require_presence_at_site_overnight"] = ["plymouth"]
 
 Rota.add_shifts(
     SingleShift(
@@ -322,7 +323,7 @@ else:
                 oop,
                 not_available_to_work=leave_requests,
                 work_requests=work_requests,
-                proc_site=site_pref,
+                night_site=site_pref,
                 previous_shifts=previous_shifts,
                 pair=pair,
                 shift_balance_extra=w["shift_balance_extra"],
diff --git a/shifts.py b/shifts.py
index 3d6b3c4..6ba6a24 100644
--- a/shifts.py
+++ b/shifts.py
@@ -181,6 +181,8 @@ class RotaBuilder(object):
             "prevent_monday_and_tuesday_after_full_weekends": [],
             "prevent_fridays_before_full_weekends": [],
             "prevent_thursdays_before_full_weekends": [],
+            # This may be better defined on the shift?
+            "require_presence_at_site_overnight": [],
             "avoid_st2_first_month": False,
             "hard_constrain_pair_separation": False,
         }
@@ -739,24 +741,47 @@ class RotaBuilder(object):
                 rule=nightShiftMinST4Rule,
             )
 
-        def nightShiftDerrifordRule(model, week, shift):
-            derriford_workers = [w for w in self.workers if w.night_at_derriford >= 1]
-            # if not derriford_workers:
-            # return Constraint.Skip
+        # def nightShiftDerrifordRule(model, week, shift):
+        #    derriford_workers = [w for w in self.workers if w.night_at_derriford >= 1]
+        #    # if not derriford_workers:
+        #    # return Constraint.Skip
+        #    return (
+        #        sum(
+        #            model.shift_week_worker_assigned[shift, week, w.id]
+        #            for w in derriford_workers
+        #        )
+        #        >= 1
+        #    )
+
+        # if self.constraint_options["ensure_derriford_reg_for_nights"]:
+        #    self.model.night_shifts_derriford_rule = Constraint(
+        #        [week for week in self.weeks],
+        #        [shift.name for shift in self.get_shifts_with_constraint("night")],
+        #        rule=nightShiftDerrifordRule,
+        #    )
+
+        def presenceAtSiteOvernight(model, week, shift, required_site):
+            required_site_workers = [
+                w for w in self.workers if required_site == w.night_site
+            ]
+
             return (
                 sum(
                     model.shift_week_worker_assigned[shift, week, w.id]
-                    for w in derriford_workers
+                    for w in required_site_workers
                 )
                 >= 1
             )
 
-        if self.constraint_options["ensure_derriford_reg_for_nights"]:
-            self.model.night_shifts_derriford_rule = Constraint(
-                [week for week in self.weeks],
-                [shift.name for shift in self.get_shifts_with_constraint("night")],
-                rule=nightShiftDerrifordRule,
-            )
+        if self.constraint_options["require_presence_at_site_overnight"]:
+
+            for site in self.constraint_options["require_presence_at_site_overnight"]:
+                self.model.require_presence_at_site_overnight_rule = Constraint(
+                    [week for week in self.weeks],
+                    [shift.name for shift in self.get_shifts_with_constraint("night")],
+                    [site],
+                    rule=presenceAtSiteOvernight,
+                )
 
         # # Count the number of workers from each site on each night shift
         # # As 1 or 0 is optimum we can simply subtract 1 from the number
diff --git a/workers.py b/workers.py
index 42d10d3..c7ade52 100644
--- a/workers.py
+++ b/workers.py
@@ -20,7 +20,7 @@ class Worker:
         not_available_to_work=None,
         pref_not_to_work=None,
         work_requests=None,
-        proc_site: str = "plymouth",  # We set a default proc_site
+        night_site: str = "plymouth",  # We set a default proc_site
         previous_shifts: dict = {},
         shift_balance_extra: dict = {},
         bank_holiday_extra: int = 0,
@@ -44,8 +44,8 @@ class Worker:
         self.shift_balance_extra = shift_balance_extra
         self.bank_holiday_extra = bank_holiday_extra
 
-        self.proc_site = proc_site
-        if proc_site == "plymouth" or proc_site == "":
+        self.proc_site = night_site
+        if night_site == "plymouth" or night_site == "":
             self.night_at_derriford = 1
         else:
             self.night_at_derriford = 0