diff --git a/src/prolog/clause_types.rs b/src/prolog/clause_types.rs
index a31eba1a..98a6d902 100644
--- a/src/prolog/clause_types.rs
+++ b/src/prolog/clause_types.rs
@@ -251,6 +251,7 @@ pub enum SystemClauseType {
     SetDoubleQuotes,
     SetSeed,
     SkipMaxList,
+    Sleep,
     Succeed,
     TermVariables,
     TruncateLiftedHeapTo,
@@ -287,16 +288,16 @@ impl SystemClauseType {
             &SystemClauseType::CurrentInput => clause_name!("$current_input"),
             &SystemClauseType::CurrentOutput => clause_name!("$current_output"),
             &SystemClauseType::REPL(REPLCodePtr::CompileBatch) => clause_name!("$compile_batch"),
-	        &SystemClauseType::REPL(REPLCodePtr::UseModule) => clause_name!("$use_module"),
-	        &SystemClauseType::REPL(REPLCodePtr::UseQualifiedModule) => {
-		        clause_name!("$use_qualified_module")
-	        }
-	        &SystemClauseType::REPL(REPLCodePtr::UseModuleFromFile) => {
-		        clause_name!("$use_module_from_file")
-	        }
-	        &SystemClauseType::REPL(REPLCodePtr::UseQualifiedModuleFromFile) => {
-		        clause_name!("$use_qualified_module_from_file")
-	        }
+            &SystemClauseType::REPL(REPLCodePtr::UseModule) => clause_name!("$use_module"),
+            &SystemClauseType::REPL(REPLCodePtr::UseQualifiedModule) => {
+                    clause_name!("$use_qualified_module")
+            }
+            &SystemClauseType::REPL(REPLCodePtr::UseModuleFromFile) => {
+                    clause_name!("$use_module_from_file")
+            }
+            &SystemClauseType::REPL(REPLCodePtr::UseQualifiedModuleFromFile) => {
+                    clause_name!("$use_qualified_module_from_file")
+            }
             &SystemClauseType::CopyToLiftedHeap => clause_name!("$copy_to_lh"),
             &SystemClauseType::DeleteAttribute => clause_name!("$del_attr_non_head"),
             &SystemClauseType::DeleteHeadAttribute => clause_name!("$del_attr_head"),
@@ -402,6 +403,7 @@ impl SystemClauseType {
             &SystemClauseType::SetCutPointByDefault(_) => clause_name!("$set_cp_by_default"),
             &SystemClauseType::SetDoubleQuotes => clause_name!("$set_double_quotes"),
             &SystemClauseType::SkipMaxList => clause_name!("$skip_max_list"),
+            &SystemClauseType::Sleep => clause_name!("$sleep"),
             &SystemClauseType::Succeed => clause_name!("$succeed"),
             &SystemClauseType::TermVariables => clause_name!("$term_variables"),
             &SystemClauseType::TruncateLiftedHeapTo => clause_name!("$truncate_lh_to"),
@@ -525,6 +527,7 @@ impl SystemClauseType {
             ("$set_double_quotes", 1) => Some(SystemClauseType::SetDoubleQuotes),
             ("$set_seed", 1) => Some(SystemClauseType::SetSeed),
             ("$skip_max_list", 4) => Some(SystemClauseType::SkipMaxList),
+            ("$sleep", 1) => Some(SystemClauseType::Sleep),
             ("$store_global_var", 2) => Some(SystemClauseType::StoreGlobalVar),
             ("$store_global_var_with_offset", 2) => Some(SystemClauseType::StoreGlobalVarWithOffset),
             ("$term_variables", 2) => Some(SystemClauseType::TermVariables),
@@ -532,13 +535,13 @@ impl SystemClauseType {
             ("$unwind_environments", 0) => Some(SystemClauseType::UnwindEnvironments),
             ("$unwind_stack", 0) => Some(SystemClauseType::UnwindStack),
             ("$unify_with_occurs_check", 2) => Some(SystemClauseType::UnifyWithOccursCheck),
-	        ("$use_module", 1) => Some(SystemClauseType::REPL(REPLCodePtr::UseModule)),
-	        ("$use_module_from_file", 1) =>
-		        Some(SystemClauseType::REPL(REPLCodePtr::UseModuleFromFile)),
-	        ("$use_qualified_module", 2) =>
-		        Some(SystemClauseType::REPL(REPLCodePtr::UseQualifiedModule)),
-	        ("$use_qualified_module_from_file", 2) =>
-		        Some(SystemClauseType::REPL(REPLCodePtr::UseQualifiedModuleFromFile)),
+            ("$use_module", 1) => Some(SystemClauseType::REPL(REPLCodePtr::UseModule)),
+            ("$use_module_from_file", 1) =>
+                    Some(SystemClauseType::REPL(REPLCodePtr::UseModuleFromFile)),
+            ("$use_qualified_module", 2) =>
+                    Some(SystemClauseType::REPL(REPLCodePtr::UseQualifiedModule)),
+            ("$use_qualified_module_from_file", 2) =>
+                    Some(SystemClauseType::REPL(REPLCodePtr::UseQualifiedModuleFromFile)),
             ("$variant", 2) => Some(SystemClauseType::Variant),
             ("$wam_instructions", 3) => Some(SystemClauseType::WAMInstructions),
             ("$write_term", 6) => Some(SystemClauseType::WriteTerm),
diff --git a/src/prolog/lib/time.pl b/src/prolog/lib/time.pl
index 97358a7d..b41d958a 100644
--- a/src/prolog/lib/time.pl
+++ b/src/prolog/lib/time.pl
@@ -3,17 +3,26 @@
    Part of Scryer Prolog.
 
    This library provides predicates for reasoning about time.
-   sleep/1 should be implemented here, sleeping for a number of seconds.
-   In addition, this library should provide reasoning about time stamps.
+   Reasoning about time stamps would be a useful addition, for example
+   by obtaining the current time, comparing and formatting it.
 
    '$cpu_new' can be replaced by statistics/2 once that is implemented.
 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
 
-:- module(time, [time/1]).
+:- module(time, [sleep/1, time/1]).
 
 :- use_module(library(format)).
 :- use_module(library(iso_ext)).
 
+sleep(T) :-
+    builtins:must_be_number(T, sleep),
+    (   T < 0 ->
+        throw(domain_error(not_less_than_zero, T))
+    ;   T > 0xfffffffffffffbff ->
+        throw(domain_error(not_great_than_0xfffffffffffffbff, T))
+    ;   '$sleep'(T)
+    ).
+
 time(Goal) :-
         '$cpu_now'(T0),
         setup_call_cleanup(true,
diff --git a/src/prolog/machine/system_calls.rs b/src/prolog/machine/system_calls.rs
index d838aadf..a9b5688a 100644
--- a/src/prolog/machine/system_calls.rs
+++ b/src/prolog/machine/system_calls.rs
@@ -3092,6 +3092,22 @@ impl MachineState {
                 if let Err(err) = self.skip_max_list() {
                     return Err(err);
                 },
+            &SystemClauseType::Sleep => {
+                let time = self.store(self.deref(self[temp_v!(1)]));
+
+                let time = match Number::try_from((time, &self.heap)) {
+                    Ok(Number::Float(OrderedFloat(n))) => n,
+                    Ok(Number::Fixnum(n)) => n as f64,
+                    Ok(Number::Integer(n)) => n.to_f64(),
+                    _ => {
+                        unreachable!()
+                    }
+                };
+
+                let duration = Duration::new(1, 0);
+                let duration = duration.mul_f64(time);
+                ::std::thread::sleep(duration);
+            }
             &SystemClauseType::StoreGlobalVar => {
                 let key = self[temp_v!(1)];