diff --git a/usr/testbed/test.cpp b/usr/testbed/test.cpp
index ef3f2eb..3f7f324 100644
--- a/usr/testbed/test.cpp
+++ b/usr/testbed/test.cpp
@@ -1,7 +1,10 @@
 #include <mammoth/util/debug.h>
+#include <zcall.h>
 
 uint64_t main(uint64_t init_port_cap) {
   dbgln("testbed");
+  check(ZThreadSleep(2000));
+  dbgln("testbed2");
 
   return glcr::OK;
 }
diff --git a/zion/include/zcall.h b/zion/include/zcall.h
index efe5d16..c5f9ca6 100644
--- a/zion/include/zcall.h
+++ b/zion/include/zcall.h
@@ -17,6 +17,7 @@ SYS4(ThreadStart, z_cap_t, thread_cap, uint64_t, entry, uint64_t, arg1,
      uint64_t, arg2);
 SYS0(ThreadExit);
 SYS1(ThreadWait, z_cap_t, thread_cap);
+SYS1(ThreadSleep, uint64_t, millis);
 
 SYS5(AddressSpaceMap, z_cap_t, vmas_cap, uint64_t, vmas_offset, z_cap_t,
      vmmo_cap, uint64_t, align, uint64_t*, vaddr);
diff --git a/zion/include/ztypes.h b/zion/include/ztypes.h
index f25df71..490e0d7 100644
--- a/zion/include/ztypes.h
+++ b/zion/include/ztypes.h
@@ -19,6 +19,7 @@ const uint64_t kZionThreadCreate = 0x10;
 const uint64_t kZionThreadStart = 0x11;
 const uint64_t kZionThreadExit = 0x12;
 const uint64_t kZionThreadWait = 0x13;
+const uint64_t kZionThreadSleep = 0x14;
 
 // Memory Calls
 const uint64_t kZionAddressSpaceMap = 0x21;
diff --git a/zion/object/thread.h b/zion/object/thread.h
index 53bee2b..6c32007 100644
--- a/zion/object/thread.h
+++ b/zion/object/thread.h
@@ -29,6 +29,7 @@ class Thread : public KernelObject, public glcr::IntrusiveListNode<Thread> {
     RUNNING,
     RUNNABLE,
     BLOCKED,
+    SLEEPING,
     CLEANUP,
     FINISHED,
   };
@@ -69,6 +70,9 @@ class Thread : public KernelObject, public glcr::IntrusiveListNode<Thread> {
 
   void Wait();
 
+  void SetSleepTicks(uint64_t sleep_ticks) { sleep_ticks_ = sleep_ticks; }
+  bool DecrementSleepTicks() { return --sleep_ticks_ == 0; }
+
  private:
   friend class glcr::MakeRefCountedFriend<Thread>;
   Thread(Process& proc, uint64_t tid);
@@ -79,6 +83,7 @@ class Thread : public KernelObject, public glcr::IntrusiveListNode<Thread> {
   State state_ = CREATED;
   bool is_kernel_ = false;
   uint64_t user_stack_base_;
+  uint64_t sleep_ticks_;
 
   // Startup Context for the thread.
   uint64_t rip_;
diff --git a/zion/scheduler/scheduler.cpp b/zion/scheduler/scheduler.cpp
index 5d745c8..2a21d30 100644
--- a/zion/scheduler/scheduler.cpp
+++ b/zion/scheduler/scheduler.cpp
@@ -45,14 +45,11 @@ void Scheduler::Preempt() {
     return;
   }
 
+  DecrementSleepingThreads();
+
   ClearDeadThreadsFromFront();
 
   asm volatile("cli");
-  if (current_thread_ == sleep_thread_) {
-    // Sleep should never be preempted. (We should yield it if another thread
-    // becomes scheduleable).
-    return;
-  }
 
   if (runnable_threads_.size() == 0) {
     // Continue.
@@ -102,9 +99,33 @@ void Scheduler::Yield() {
   SwapToCurrent(*prev);
 }
 
+void Scheduler::Sleep(uint64_t millis) {
+  // FIXME: Improve resolution of sleep calls.
+  uint64_t ticks = (millis / 50) + 1;
+  current_thread_->SetSleepTicks(ticks);
+  current_thread_->SetState(Thread::SLEEPING);
+  sleeping_threads_.PushBack(current_thread_);
+  Yield();
+}
+
 void Scheduler::ClearDeadThreadsFromFront() {
   while (runnable_threads_.size() > 0 &&
          runnable_threads_.PeekFront()->IsDying()) {
     runnable_threads_.PopFront();
   }
 }
+
+void Scheduler::DecrementSleepingThreads() {
+  auto thread = sleeping_threads_.PeekFront();
+  while (thread) {
+    if (thread->DecrementSleepTicks()) {
+      auto thread_next = thread->next_;
+      sleeping_threads_.Remove(thread);
+      thread->SetState(Thread::RUNNABLE);
+      runnable_threads_.PushBack(thread);
+      thread = thread_next;
+    } else {
+      thread = thread->next_;
+    }
+  }
+}
diff --git a/zion/scheduler/scheduler.h b/zion/scheduler/scheduler.h
index 823ded8..37aa0b9 100644
--- a/zion/scheduler/scheduler.h
+++ b/zion/scheduler/scheduler.h
@@ -23,18 +23,23 @@ class Scheduler {
   void Preempt();
   void Yield();
 
+  void Sleep(uint64_t millis);
+
  private:
   bool enabled_ = false;
 
   glcr::RefPtr<Thread> current_thread_;
   glcr::IntrusiveList<Thread> runnable_threads_;
 
+  glcr::IntrusiveList<Thread> sleeping_threads_;
+
   glcr::RefPtr<Thread> sleep_thread_;
 
   Scheduler();
   void SwapToCurrent(Thread& prev);
 
   void ClearDeadThreadsFromFront();
+  void DecrementSleepingThreads();
 };
 
 extern Scheduler* gScheduler;
diff --git a/zion/syscall/syscall.cpp b/zion/syscall/syscall.cpp
index ac260a5..70e9677 100644
--- a/zion/syscall/syscall.cpp
+++ b/zion/syscall/syscall.cpp
@@ -59,6 +59,7 @@ extern "C" z_err_t SyscallHandler(uint64_t call_id, void* req) {
     CASE(ThreadStart);
     CASE(ThreadExit);
     CASE(ThreadWait);
+    CASE(ThreadSleep);
     // syscall/address_space.h
     CASE(AddressSpaceMap);
     CASE(AddressSpaceUnmap);
diff --git a/zion/syscall/thread.cpp b/zion/syscall/thread.cpp
index 36ca2fe..19f212e 100644
--- a/zion/syscall/thread.cpp
+++ b/zion/syscall/thread.cpp
@@ -53,3 +53,8 @@ glcr::ErrorCode ThreadWait(ZThreadWaitReq* req) {
   thread->Wait();
   return glcr::OK;
 }
+
+glcr::ErrorCode ThreadSleep(ZThreadSleepReq* req) {
+  gScheduler->Sleep(req->millis);
+  return glcr::OK;
+}
diff --git a/zion/syscall/thread.h b/zion/syscall/thread.h
index 17cc288..4abd323 100644
--- a/zion/syscall/thread.h
+++ b/zion/syscall/thread.h
@@ -8,3 +8,4 @@ glcr::ErrorCode ThreadCreate(ZThreadCreateReq* req);
 glcr::ErrorCode ThreadStart(ZThreadStartReq* req);
 glcr::ErrorCode ThreadExit(ZThreadExitReq*);
 glcr::ErrorCode ThreadWait(ZThreadWaitReq* req);
+glcr::ErrorCode ThreadSleep(ZThreadSleepReq* req);