diff --git a/internal/cmd/sleep.go b/internal/cmd/sleep.go
index 8a22ff1..363a903 100644
--- a/internal/cmd/sleep.go
+++ b/internal/cmd/sleep.go
@@ -28,10 +28,7 @@ var sleepDayCmd = &cobra.Command{
 		if date == "" {
 			date = time.Now().Format("2006-01-02")
 		}
-		tz := viper.GetString("timezone")
-		if tz == "local" {
-			tz = time.Local.String()
-		}
+		tz := resolveTimezone(viper.GetString("timezone"))
 		cl := client.New(viper.GetString("email"), viper.GetString("password"), viper.GetString("user_id"), viper.GetString("client_id"), viper.GetString("client_secret"))
 		day, err := cl.GetSleepDay(context.Background(), date, tz)
 		if err != nil {
diff --git a/internal/cmd/sleep_range.go b/internal/cmd/sleep_range.go
index d31f1ec..e5c86b9 100644
--- a/internal/cmd/sleep_range.go
+++ b/internal/cmd/sleep_range.go
@@ -36,10 +36,7 @@ var sleepRangeCmd = &cobra.Command{
 		if end.Before(start) {
 			return fmt.Errorf("to must be >= from")
 		}
-		tz := viper.GetString("timezone")
-		if tz == "local" {
-			tz = time.Local.String()
-		}
+		tz := resolveTimezone(viper.GetString("timezone"))
 		cl := client.New(viper.GetString("email"), viper.GetString("password"), viper.GetString("user_id"), viper.GetString("client_id"), viper.GetString("client_secret"))
 		rows := []map[string]any{}
 		for d := start; !d.After(end); d = d.Add(24 * time.Hour) {
diff --git a/internal/cmd/timezone.go b/internal/cmd/timezone.go
new file mode 100644
index 0000000..fc87778
--- /dev/null
+++ b/internal/cmd/timezone.go
@@ -0,0 +1,51 @@
+package cmd
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+// resolveTimezone converts a timezone string to a valid IANA timezone name.
+// Go's time.Local.String() returns "Local" which is not a valid IANA name
+// and is rejected by the Eight Sleep API.
+func resolveTimezone(tz string) string {
+	if tz != "" && tz != "local" && tz != "Local" {
+		return tz
+	}
+	if iana := localIANA(); iana != "" {
+		return iana
+	}
+	return tz
+}
+
+// localIANA detects the IANA timezone name from the operating system.
+func localIANA() string {
+	if tz := os.Getenv("TZ"); tz != "" && tz != "Local" {
+		return tz
+	}
+	switch runtime.GOOS {
+	case "darwin":
+		// macOS: /etc/localtime is a symlink into zoneinfo
+		target, err := os.Readlink("/etc/localtime")
+		if err == nil {
+			if idx := strings.Index(target, "zoneinfo/"); idx >= 0 {
+				return target[idx+len("zoneinfo/"):]
+			}
+		}
+	case "linux":
+		if b, err := os.ReadFile("/etc/timezone"); err == nil {
+			if tz := strings.TrimSpace(string(b)); tz != "" {
+				return tz
+			}
+		}
+		target, err := filepath.EvalSymlinks("/etc/localtime")
+		if err == nil {
+			if idx := strings.Index(target, "zoneinfo/"); idx >= 0 {
+				return target[idx+len("zoneinfo/"):]
+			}
+		}
+	}
+	return ""
+}