diff --git a/beacon_chain/beacon_node.nim b/beacon_chain/beacon_node.nim
index 81067d933..c5b840484 100644
--- a/beacon_chain/beacon_node.nim
+++ b/beacon_chain/beacon_node.nim
@@ -15,7 +15,7 @@ type
     attachedValidators: ValidatorPool
     attestationPool: AttestationPool
     mainchainMonitor: MainchainMonitor
-    lastScheduledCycle: int
+    lastScheduledEpoch: uint64
     headBlock: BeaconBlock
     headBlockRoot: Eth2Digest
 
@@ -77,7 +77,7 @@ proc connectToNetwork(node: BeaconNode) {.async.} =
 proc sync*(node: BeaconNode): Future[bool] {.async.} =
   let persistedState = node.db.lastFinalizedState()
   if persistedState.isNil or
-     persistedState[].slotDistanceFromNow() > WEAK_SUBJECTVITY_PERIOD:
+     persistedState[].slotDistanceFromNow() > WEAK_SUBJECTVITY_PERIOD.int64:
     if node.config.stateSnapshot.isSome:
       node.beaconState = node.config.stateSnapshot.get
     else:
@@ -86,7 +86,7 @@ proc sync*(node: BeaconNode): Future[bool] {.async.} =
     node.beaconState = persistedState[]
     var targetSlot = toSlot timeSinceGenesis(node.beaconState)
 
-    while node.beaconState.finalized_slot.int < targetSlot:
+    while node.beaconState.finalized_slot < targetSlot:
       var (peer, changeLog) = await node.network.getValidatorChangeLog(
         node.beaconState.validator_registry_delta_chain_tip)
 
@@ -218,11 +218,11 @@ proc proposeBlock(node: BeaconNode,
                          validator = validator.idx
 
 proc scheduleBlockProposal(node: BeaconNode,
-                           slot: int,
+                           slot: uint64,
                            validator: AttachedValidator) =
   # TODO:
   # This function exists only to hide a bug with Nim's closures.
-  # If you inline it in `scheduleCycleActions`, you'll see the
+  # If you inline it in `scheduleEpochActions`, you'll see the
   # internal `doAssert` starting to fail.
   doAssert validator != nil
 
@@ -232,13 +232,13 @@ proc scheduleBlockProposal(node: BeaconNode,
 
 proc scheduleAttestation(node: BeaconNode,
                          validator: AttachedValidator,
-                         slot: int,
+                         slot: uint64,
                          shard: uint64,
                          committeeLen: int,
                          indexInCommittee: int) =
   # TODO:
   # This function exists only to hide a bug with Nim's closures.
-  # If you inline it in `scheduleCycleActions`, you'll see the
+  # If you inline it in `scheduleEpochActions`, you'll see the
   # internal `doAssert` starting to fail.
   doAssert validator != nil
 
@@ -247,7 +247,7 @@ proc scheduleAttestation(node: BeaconNode,
     asyncCheck makeAttestation(node, validator, slot.uint64,
                                shard, committeeLen, indexInCommittee)
 
-proc scheduleCycleActions(node: BeaconNode, cycleStart: int) =
+proc scheduleEpochActions(node: BeaconNode, epoch: uint64) =
   ## This schedules the required block proposals and
   ## attestations from our attached validators.
   doAssert node != nil
@@ -256,14 +256,12 @@ proc scheduleCycleActions(node: BeaconNode, cycleStart: int) =
   # see the comments in `get_beacon_proposer_index`
   var nextState = node.beaconState
 
-  for i in 1 ..< EPOCH_LENGTH:
+  for i in 1.uint64 ..< EPOCH_LENGTH:
     # Schedule block proposals
-    nextState.slot = node.beaconState.slot + i.uint64
-
-    let
-      slot = cycleStart + i
-      proposerIdx = get_beacon_proposer_index(nextState, nextState.slot.uint64)
-      validator = node.getAttachedValidator(proposerIdx)
+    let slot = epoch * EPOCH_LENGTH + i
+    nextState.slot = slot
+    let proposerIdx = get_beacon_proposer_index(nextState, slot)
+    let validator = node.getAttachedValidator(proposerIdx)
 
     if validator != nil:
       # TODO:
@@ -273,7 +271,7 @@ proc scheduleCycleActions(node: BeaconNode, cycleStart: int) =
 
     # Schedule attestations
     let
-      committeesIdx = get_shard_committees_index(nextState, nextState.slot.uint64)
+      committeesIdx = get_shard_committees_index(nextState, slot)
 
     for shard in node.beaconState.shard_committees_at_slots[committees_idx]:
       for i, validatorIdx in shard.committee:
@@ -281,12 +279,12 @@ proc scheduleCycleActions(node: BeaconNode, cycleStart: int) =
         if validator != nil:
           scheduleAttestation(node, validator, slot, shard.shard, shard.committee.len, i)
 
-  node.lastScheduledCycle = cycleStart
-  let nextCycle = cycleStart + EPOCH_LENGTH
+  node.lastScheduledEpoch = epoch
+  let nextEpoch = epoch + 1
 
-  addTimer(node.beaconState.slotMiddle(nextCycle)) do (p: pointer):
-    if node.lastScheduledCycle != nextCycle:
-      node.scheduleCycleActions(nextCycle)
+  addTimer(node.beaconState.slotMiddle(nextEpoch * EPOCH_LENGTH)) do (p: pointer):
+    if node.lastScheduledEpoch != nextEpoch:
+      node.scheduleEpochActions(nextEpoch)
 
 proc processBlocks*(node: BeaconNode) =
   node.network.subscribe(topicBeaconBlocks) do (newBlock: BeaconBlock):
@@ -314,10 +312,9 @@ proc processBlocks*(node: BeaconNode) =
     # 3. Peform block processing / state recalculation / etc
     #
 
-    let slot = newBlock.slot.int
-    if slot mod EPOCH_LENGTH == 0:
-      node.scheduleCycleActions(slot)
-      node.attestationPool.discardHistoryToSlot(slot)
+    let epoch = newBlock.slot.epoch
+    if epoch != node.lastScheduledEpoch:
+      node.scheduleEpochActions(epoch)
 
   node.network.subscribe(topicAttestations) do (a: Attestation):
     info "Attestation received", slot = a.data.slot,
@@ -327,8 +324,8 @@ proc processBlocks*(node: BeaconNode) =
     node.attestationPool.add(a, node.beaconState)
 
   dynamicLogScope(node = node.config.tcpPort - 50000):
-    let cycleStart = node.beaconState.slot.int
-    node.scheduleCycleActions(cycleStart)
+    let epoch = node.beaconState.slot.epoch
+    node.scheduleEpochActions(epoch)
 
     runForever()
 
diff --git a/beacon_chain/fork_choice.nim b/beacon_chain/fork_choice.nim
index 80e834730..1456d3d37 100644
--- a/beacon_chain/fork_choice.nim
+++ b/beacon_chain/fork_choice.nim
@@ -17,6 +17,14 @@ type
     attestations: Deque[array[SHARD_COUNT, Option[Attestation]]]
     startingSlot: int
 
+  # TODO:
+  # The compilicated Deque above is not needed.
+  #
+  # In fact, we can use a simple array with length SHARD_COUNT because
+  # in each epoch, each shard is going to receive attestations exactly once.
+  # Once the epoch is over, we can discard all attestations and start all
+  # over again (no need for `discardHistoryToSlot` too).
+
 proc init*(T: type AttestationPool, startingSlot: int): T =
   result.attestations = initDeque[array[SHARD_COUNT, Option[Attestation]]]()
   result.startingSlot = startingSlot
diff --git a/beacon_chain/spec/datatypes.nim b/beacon_chain/spec/datatypes.nim
index 9510bd8a3..4f384ff46 100644
--- a/beacon_chain/spec/datatypes.nim
+++ b/beacon_chain/spec/datatypes.nim
@@ -94,7 +94,7 @@ const
   ZERO_HASH* = Eth2Digest()
 
   # Time constants
-  SLOT_DURATION* = 6 ## \
+  SLOT_DURATION* = 6'u64 ## \
   ## TODO consistent time unit across projects, similar to C++ chrono?
 
   MIN_ATTESTATION_INCLUSION_DELAY* = 2'u64^2 ##\
@@ -415,6 +415,9 @@ type
     DOMAIN_PROPOSAL = 2
     DOMAIN_EXIT = 3
 
+template epoch*(slot: int|uint64): auto =
+  slot div EPOCH_LENGTH
+
 when true:
   # TODO: Remove these once RLP serialization is no longer used
   import nimcrypto, rlp, json_serialization
diff --git a/beacon_chain/time.nim b/beacon_chain/time.nim
index f909d6cd5..6b294b969 100644
--- a/beacon_chain/time.nim
+++ b/beacon_chain/time.nim
@@ -15,20 +15,20 @@ proc timeSinceGenesis*(s: BeaconState): Timestamp =
   Timestamp(int64(fastEpochTime() - s.genesis_time * 1000) -
             detectedClockDrift)
 
-template toSlot*(t: Timestamp): int =
-  int(t div uint64(SLOT_DURATION * 1000))
+template toSlot*(t: Timestamp): uint64 =
+  t div (SLOT_DURATION * 1000)
 
-template slotStart*(s: BeaconState, slot: int): Timestamp =
-  (s.genesis_time + uint64(slot * SLOT_DURATION)) * 1000
+template slotStart*(s: BeaconState, slot: uint64): Timestamp =
+  (s.genesis_time + (slot * SLOT_DURATION)) * 1000
 
-template slotMiddle*(s: BeaconState, slot: int): Timestamp =
+template slotMiddle*(s: BeaconState, slot: uint64): Timestamp =
   s.slotStart(slot) + SLOT_DURATION * 500
 
-template slotEnd*(s: BeaconState, slot: int): Timestamp =
+template slotEnd*(s: BeaconState, slot: uint64): Timestamp =
   s.slotStart(slot + 1)
 
 proc randomTimeInSlot*(s: BeaconState,
-                       slot: Natural,
+                       slot: uint64,
                        interval: HSlice[float, float]): Timestamp =
   ## Returns a random moment within the slot.
   ## The interval must be a sub-interval of [0..1].
diff --git a/beacon_chain/trusted_state_snapshots.nim b/beacon_chain/trusted_state_snapshots.nim
index 310b03bec..c6ed9fcf2 100644
--- a/beacon_chain/trusted_state_snapshots.nim
+++ b/beacon_chain/trusted_state_snapshots.nim
@@ -3,7 +3,7 @@ import
   spec/[datatypes, crypto, digest, beaconstate], beacon_chain_db, conf
 
 const
-  WEAK_SUBJECTVITY_PERIOD* = 4 * 30 * 24 * 60 * 60 div SLOT_DURATION
+  WEAK_SUBJECTVITY_PERIOD* = uint64(4 * 30 * 24 * 60 * 60) div SLOT_DURATION
     # TODO: This needs revisiting.
     # Why was the validator WITHDRAWAL_PERIOD altered in the spec?
 
diff --git a/beacon_chain/validator_pool.nim b/beacon_chain/validator_pool.nim
index 7aa45dfaf..461ffe66b 100644
--- a/beacon_chain/validator_pool.nim
+++ b/beacon_chain/validator_pool.nim
@@ -50,7 +50,7 @@ proc signBlockProposal*(v: AttachedValidator,
     let proposalRoot = hash_tree_root_final(proposal)
 
     # TODO: Should we use proposalRoot as data, or digest in regards to signature?
-    return signMessage(v.privKey, proposalRoot.data)
+    result = signMessage(v.privKey, proposalRoot.data)
   else:
     # TODO:
     # send RPC
@@ -64,7 +64,8 @@ proc signAttestation*(v: AttachedValidator,
 
     let attestationRoot = hash_tree_root_final(attestation)
     # TODO: Avoid the allocations belows
-    return signMessage(v.privKey, @(attestationRoot.data) & @[0'u8])
+    var dataToSign = @(attestationRoot.data) & @[0'u8]
+    result = signMessage(v.privKey, dataToSign)
   else:
     # TODO:
     # send RPC
diff --git a/tests/simulation/start.sh b/tests/simulation/start.sh
index 5a973fee8..3bb632999 100755
--- a/tests/simulation/start.sh
+++ b/tests/simulation/start.sh
@@ -13,15 +13,20 @@ SNAPSHOT_FILE="$SIMULATION_DIR/state_snapshot.json"
 cd $(git rev-parse --show-toplevel)
 ROOT_DIR=$PWD
 
-nim c beacon_chain/validator_keygen
-nim c beacon_chain/beacon_node
+BEACON_NODE_BIN=$BUILD_OUTPUTS_DIR/beacon_node
+VALIDATOR_KEYGEN_BIN=$BUILD_OUTPUTS_DIR/validator_keygen
+
+if [[ ! -z "$SKIP_BUILDS" ]]; then
+  nim c -o:"$VALIDATOR_KEYGEN_BIN" beacon_chain/validator_keygen
+  nim c -o:"$BEACON_NODE_BIN" beacon_chain/beacon_node
+fi
 
 if [ ! -f $STARTUP_FILE ]; then
-  beacon_chain/validator_keygen $NUMBER_OF_VALIDATORS "$SIMULATION_DIR"
+  $VALIDATOR_KEYGEN_BIN $NUMBER_OF_VALIDATORS "$SIMULATION_DIR"
 fi
 
 if [ ! -f $SNAPSHOT_FILE ]; then
-  beacon_chain/beacon_node createChain \
+  $BEACON_NODE_BIN createChain \
     --chainStartupData:$STARTUP_FILE \
     --out:$SNAPSHOT_FILE
 fi
@@ -47,7 +52,7 @@ for i in $(seq 0 9); do
 
   DATA_DIR=$SIMULATION_DIR/data-$i
 
-  beacon_chain/beacon_node \
+  $BEACON_NODE_BIN \
     --dataDir:"$DATA_DIR" \
     --validator:"$SIMULATION_DIR/validator-${i}1.json" \
     --validator:"$SIMULATION_DIR/validator-${i}2.json" \
@@ -64,3 +69,12 @@ for i in $(seq 0 9); do
     $BOOTSTRAP_NODES_FLAG &
 done
 
+trap ctrl_c INT
+
+function ctrl_c() {
+  killall beacon_node
+  exit 0
+}
+
+sleep 100000
+