diff --git a/schedule.go b/schedule.go
index b12c18a..331c69b 100644
--- a/schedule.go
+++ b/schedule.go
@@ -1,13 +1,18 @@
 package schedule
 
-import "time"
+import (
+	"sync"
+	"sync/atomic"
+	"time"
+)
 
 // Task holds information about the running task and can be used to stop running tasks.
 type Task struct {
 	stop          chan struct{}
 	nextExecution time.Time
 	startedAt     time.Time
-	stopped       bool
+	stopped       int32 // 0 means active, 1 means stopped
+	once          sync.Once
 }
 
 // newTask creates a new Task.
@@ -35,7 +40,7 @@ func (s *Task) ExecutesIn() time.Duration {
 
 // IsActive returns true if the scheduler is active.
 func (s *Task) IsActive() bool {
-	return !s.stopped
+	return atomic.LoadInt32(&s.stopped) == 0
 }
 
 // Wait blocks until the scheduler is stopped.
@@ -46,12 +51,10 @@ func (s *Task) Wait() {
 
 // Stop stops the scheduler.
 func (s *Task) Stop() {
-	if s.stopped {
-		return
-	}
-
-	s.stopped = true
-	close(s.stop)
+	s.once.Do(func() {
+		atomic.StoreInt32(&s.stopped, 1)
+		close(s.stop)
+	})
 }
 
 // After executes the task after the given duration.
@@ -59,13 +62,18 @@ func (s *Task) Stop() {
 func After(duration time.Duration, task func()) *Task {
 	scheduler := newTask()
 	scheduler.nextExecution = time.Now().Add(duration)
+	timer := time.NewTimer(duration)
 
 	go func() {
 		select {
-		case <-time.After(duration):
+		case <-timer.C:
 			task()
 			scheduler.Stop()
 		case <-scheduler.stop:
+			// If the task is stopped before the timer fires, stop the timer.
+			if !timer.Stop() {
+				<-timer.C // drain if necessary
+			}
 			return
 		}
 	}()
@@ -78,13 +86,21 @@ func After(duration time.Duration, task func()) *Task {
 func At(t time.Time, task func()) *Task {
 	scheduler := newTask()
 	scheduler.nextExecution = t
+	d := time.Until(t)
+	if d < 0 {
+		d = 0
+	}
+	timer := time.NewTimer(d)
 
 	go func() {
 		select {
-		case <-time.After(time.Until(t)):
+		case <-timer.C:
 			task()
 			scheduler.Stop()
 		case <-scheduler.stop:
+			if !timer.Stop() {
+				<-timer.C
+			}
 			return
 		}
 	}()
@@ -97,23 +113,20 @@ func At(t time.Time, task func()) *Task {
 func Every(interval time.Duration, task func() bool) *Task {
 	scheduler := newTask()
 	scheduler.nextExecution = time.Now().Add(interval)
-
 	ticker := time.NewTicker(interval)
 
 	go func() {
 		for {
 			select {
 			case <-ticker.C:
-				res := task()
-				if !res {
+				if !task() {
 					scheduler.Stop()
+					ticker.Stop()
+					return
 				}
-
 				scheduler.nextExecution = time.Now().Add(interval)
-
 			case <-scheduler.stop:
 				ticker.Stop()
-
 				return
 			}
 		}