diff --git a/beacon_chain/nimbus_beacon_node.nim b/beacon_chain/nimbus_beacon_node.nim
index c417d4dca..5f5f7435b 100644
--- a/beacon_chain/nimbus_beacon_node.nim
+++ b/beacon_chain/nimbus_beacon_node.nim
@@ -359,12 +359,11 @@ proc installAttestationSubnetHandlers(node: BeaconNode, subnets: set[uint8])
 
   await allFutures(attestationSubscriptions)
 
-proc updateStabilitySubnetMetadata(
-    node: BeaconNode, stabilitySubnets: set[uint8]) =
+proc updateStabilitySubnetMetadata(node: BeaconNode, stabilitySubnet: uint64) =
   # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/p2p-interface.md#metadata
   node.network.metadata.seq_number += 1
   for subnet in 0'u8 ..< ATTESTATION_SUBNET_COUNT:
-    node.network.metadata.attnets[subnet] = (subnet in stabilitySubnets)
+    node.network.metadata.attnets[subnet] = (subnet == stabilitySubnet)
 
   # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/validator.md#phase-0-attestation-subnet-stability
   # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/p2p-interface.md#attestation-subnet-bitfield
@@ -375,11 +374,7 @@ proc updateStabilitySubnetMetadata(
     # be the correct one and the ENR will not increase in size.
     warn "Failed to update record on subnet cycle", error = res.error
   else:
-    debug "Stability subnets changed, updated ENR attnets", stabilitySubnets
-
-func getStabilitySubnets(stabilitySubnets: auto): set[uint8] =
-  for subnetInfo in stabilitySubnets:
-    result.incl subnetInfo.subnet
+    debug "Stability subnet changed, updated ENR attnets", stabilitySubnet
 
 proc cycleAttestationSubnets(node: BeaconNode, slot: Slot) {.async.} =
   static: doAssert RANDOM_SUBNETS_PER_VALIDATOR == 1
@@ -399,8 +394,7 @@ proc cycleAttestationSubnets(node: BeaconNode, slot: Slot) {.async.} =
       node.chainDag.headState.data.data, attachedValidators,
       node.attestationSubnets, slot.epoch)
 
-  let prevStabilitySubnets =
-    getStabilitySubnets(node.attestationSubnets.stabilitySubnets)
+  let prevStabilitySubnet = node.attestationSubnets.stabilitySubnet
 
   node.attestationSubnets = newAttestationSubnets
   debug "Attestation subnets",
@@ -409,7 +403,9 @@ proc cycleAttestationSubnets(node: BeaconNode, slot: Slot) {.async.} =
       node.attestationSubnets.subscribedSubnets[1 - epochParity],
     upcoming_subnets = node.attestationSubnets.subscribedSubnets[epochParity],
     new_subnets = newSubnets,
-    stability_subnets = node.attestationSubnets.stabilitySubnets
+    stability_subnet = node.attestationSubnets.stabilitySubnet,
+    stability_subnet_expiration_epoch =
+      node.attestationSubnets.stabilitySubnetExpirationEpoch
 
   block:
     var unsubscriptions: seq[Future[void]] = @[]
@@ -421,11 +417,9 @@ proc cycleAttestationSubnets(node: BeaconNode, slot: Slot) {.async.} =
 
   await node.installAttestationSubnetHandlers(newSubnets)
 
-  let stabilitySubnets =
-    getStabilitySubnets(node.attestationSubnets.stabilitySubnets)
-
-  if stabilitySubnets != prevStabilitySubnets:
-    node.updateStabilitySubnetMetadata(stabilitySubnets)
+  let stabilitySubnet = node.attestationSubnets.stabilitySubnet
+  if stabilitySubnet != prevStabilitySubnet:
+    node.updateStabilitySubnetMetadata(stabilitySubnet)
 
 proc getAttestationSubnetHandlers(node: BeaconNode): Future[void] =
   var initialSubnets: set[uint8]
@@ -438,15 +432,11 @@ proc getAttestationSubnetHandlers(node: BeaconNode): Future[void] =
   # - Restarting the node with a presistent netkey
   # - When going from synced -> syncing -> synced state
   let wallEpoch =  node.beaconClock.now().slotOrZero().epoch
+  node.attestationSubnets.stabilitySubnet = rand(ATTESTATION_SUBNET_COUNT - 1).uint64
+  node.attestationSubnets.stabilitySubnetExpirationEpoch =
+    wallEpoch + getStabilitySubnetLength()
 
-  doAssert node.attestationSubnets.stabilitySubnets.len == 0
-  for _ in 0 ..< node.attachedValidators.count:
-    node.attestationSubnets.stabilitySubnets.add (
-      subnet: rand(ATTESTATION_SUBNET_COUNT - 1).uint8,
-      expiration: wallEpoch + getStabilitySubnetLength())
-
-  node.updateStabilitySubnetMetadata(
-    node.attestationSubnets.stabilitySubnets.getStabilitySubnets)
+  node.updateStabilitySubnetMetadata(node.attestationSubnets.stabilitySubnet)
 
   # Sets the "current" and "future" attestation subnets. One of these gets
   # replaced by get_attestation_subnet_changes() immediately.
@@ -475,8 +465,6 @@ proc removeMessageHandlers(node: BeaconNode): Future[void] =
   node.attestationSubnets.subscribedSubnets[1] = {}
   doAssert not node.getTopicSubscriptionEnabled()
 
-  node.attestationSubnets.stabilitySubnets.setLen(0)
-
   var unsubscriptions = mapIt(
     [getBeaconBlocksTopic(node.forkDigest),
      getVoluntaryExitsTopic(node.forkDigest),
diff --git a/beacon_chain/spec/datatypes.nim b/beacon_chain/spec/datatypes.nim
index deada60c9..766de946e 100644
--- a/beacon_chain/spec/datatypes.nim
+++ b/beacon_chain/spec/datatypes.nim
@@ -458,7 +458,8 @@ type
 
   AttestationSubnets* = object
     subscribedSubnets*: array[2, set[uint8]]
-    stabilitySubnets*: seq[tuple[subnet: uint8, expiration: Epoch]]
+    stabilitySubnet*: uint64
+    stabilitySubnetExpirationEpoch*: Epoch
 
   # This matches the mutable state of the Solidity deposit contract
   # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/solidity_deposit_contract/deposit_contract.sol
diff --git a/beacon_chain/spec/network.nim b/beacon_chain/spec/network.nim
index d6ced036e..64d6404b3 100644
--- a/beacon_chain/spec/network.nim
+++ b/beacon_chain/spec/network.nim
@@ -132,20 +132,12 @@ proc get_attestation_subnet_changes*(
   var attestationSubnets = prevAttestationSubnets
 
   # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/validator.md#phase-0-attestation-subnet-stability
-  var
-    prevStabilitySubnets: set[uint8] = {}
-    stabilitySet: set[uint8] = {}
-  for i in 0 ..< attestationSubnets.stabilitySubnets.len:
-    static: doAssert ATTESTATION_SUBNET_COUNT <= high(uint8)
-    prevStabilitySubnets.incl attestationSubnets.stabilitySubnets[i].subnet
-
-    if epoch >= attestationSubnets.stabilitySubnets[i].expiration:
-      attestationSubnets.stabilitySubnets[i].subnet =
-        rand(ATTESTATION_SUBNET_COUNT - 1).uint8
-      attestationSubnets.stabilitySubnets[i].expiration =
-        epoch + getStabilitySubnetLength()
-
-    stabilitySet.incl attestationSubnets.stabilitySubnets[i].subnet
+  let prevStabilitySubnet = {attestationSubnets.stabilitySubnet.uint8}
+  if epoch >= attestationSubnets.stabilitySubnetExpirationEpoch:
+    attestationSubnets.stabilitySubnet =
+      rand(ATTESTATION_SUBNET_COUNT - 1).uint64
+    attestationSubnets.stabilitySubnetExpirationEpoch =
+      epoch + getStabilitySubnetLength()
 
   var nextEpochSubnets: set[uint8]
   for it in get_committee_assignments(
@@ -156,15 +148,16 @@ proc get_attestation_subnet_changes*(
 
   let
     epochParity = epoch mod 2
+    stabilitySet = {attestationSubnets.stabilitySubnet.uint8}
     currentEpochSubnets = attestationSubnets.subscribedSubnets[1 - epochParity]
 
     expiringSubnets =
-      (prevStabilitySubnets +
+      (prevStabilitySubnet +
         attestationSubnets.subscribedSubnets[epochParity]) -
         nextEpochSubnets - currentEpochSubnets - stabilitySet
     newSubnets =
       (nextEpochSubnets + stabilitySet) -
-        (currentEpochSubnets + prevStabilitySubnets)
+        (currentEpochSubnets + prevStabilitySubnet)
 
   doAssert newSubnets.len <= attachedValidators.len + 1